C sharp systemformatexpection

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C#3.5

C#5.0

Hello
Im doing an C sharp console application, but im having an issue. Here is the warning message and the line that is marked

<br />
An unhandled exception of type 'System.FormatException' occurred in mscorlib.dll<br />
<br />
line =Convert.ToInt32(temp);

here is the full code

using System.Collections.Generic;
using System;
using System.Diagnostics;
using System.Data;
using System.Xml.Linq;
using Microsoft.VisualBasic;
using System.Collections;
using System.Linq;
 
 
using Microsoft.VisualBasic.CompilerServices;
 
namespace Assignment1
{
        sealed class Module1
        {
     
               
                static string temp; //Global variable
               
               
                static public void Main()
                {
                        int price = 0;
                        int paid = 0;
                        int balance = 0;
                       
                       
                        int no_of_500 = 0;
                        int no_of_100 = 0;
                        int no_of_50 = 0;
                        int no_of_20 = 0;
                       
                        int no_of_10 = 0;
                        int no_of_5 = 0;
                        int no_of_1 = 0;
                       
                        Console.WriteLine();
                        Console.Write("Enter the price =  ");
                       
                        //let user to enter and assigning it to variable by explicitly casing to int since option "strict on" is activated
                        price = validateInput();
                       
                        Console.WriteLine();
                        Console.Write("Paid  = ");
                        paid = validateInput();
                       
                       
                        //check paid cash is not less than the price
                        if (paid < price)
                        {
                                Console.WriteLine();
                                Console.WriteLine("Amount you paid is not sufficient.You need to pay more " + (price - paid).ToString() + " kr.");
                                Console.WriteLine();
                               
                                //show balance
                        }
                        else
                        {
                                balance = paid - price;
                                Console.WriteLine();
                                Console.Write("Balance = " + balance.ToString());
                                Console.WriteLine();
                                Console.WriteLine();
                               
                                //check for how many 500 notes
                                //if balance is greater than 500
                                //then using "\" divition find how many 500 s inside the balance
                                //finally update the balance by subtracting toatal value of 500 notes
                                //same logic will apply for all notes and coins in the order of 500,100,50,20,10,5,1
                                if (balance >= 500)
                                {
                                        no_of_500 = balance / 500;
                                        balance = balance - no_of_500 * 500;
                                }
                               
                                if (balance >= 100)
                                {
                                        no_of_100 = balance / 100;
                                        balance = balance - no_of_100 * 100;
                                }
                               
                                if (balance >= 50)
                                {
                                        no_of_50 = balance / 50;
                                        balance = balance - no_of_50 * 50;
                                }
                               
                                if (balance >= 20)
                                {
                                        no_of_20 = balance / 20;
                                        balance = balance - no_of_20 * 20;
                                }
                               
                                if (balance >= 10)
                                {
                                        no_of_10 = balance / 10;
                                        balance = balance - no_of_10 * 10;
                                }
                               
                                if (balance >= 5)
                                {
                                        no_of_5 = balance / 5;
                                        balance = balance - no_of_5 * 5;
                                }
                               
                                //at last  the remainig balance less than five means there will be equal no of 1 crown coins
                                if (balance <= 4)
                                {
                                        no_of_1 = balance;
                                }
                               
                                //show how many notes and coins to change back
                                if (no_of_500 > 0)
                                {
                                        Console.WriteLine(no_of_500.ToString() + " x 500 sek note");
                                }
                                if (no_of_100 > 0)
                                {
                                        Console.WriteLine(no_of_100.ToString() + " x 100 sek note");
                                }
                                if (no_of_50 > 0)
                                {
                                        Console.WriteLine(no_of_50.ToString() + " x 50 sek note");
                                }
                                if (no_of_20 > 0)
                                {
                                        Console.WriteLine(no_of_20.ToString() + " x 20 sek note");
                                }
                                if (no_of_10 > 0)
                                {
                                        Console.WriteLine(no_of_10.ToString() + " x 10 kr");
                                }
                                if (no_of_5 > 0)
                                {
                                        Console.WriteLine(no_of_5.ToString() + " x 5 kr");
                                }
                                if (no_of_1 > 0)
                                {
                                        Console.WriteLine(no_of_1.ToString() + " x 1 kr");
                                }
                               
                               
                        }
                       
                        //wait for key press to stop sudden closing
                        Console.ReadLine();
                       
                }
               
                private static int validateInput()
                {
                       
                        int value = default(int);
                        try
                        {
                               
                                //take no of items to string num
                                temp = Console.ReadLine();
                               
                                //if input is not numeric this will throw an InvalidCastException when it tries to cast to integer and
                //will jump to catch block
                 value =Convert.ToInt32(temp);
 
 
                //check whether is it has decimal places by checking for (".")
                                //if input is like  12.5  it will split in to two parts as 12 and 5 by "."
                                //and length of parts() will be two. that means entered value is not a whole number
                                string[] parts = temp.Split('.');
                                if (parts.Length == 2)
                                {
                                        Console.WriteLine();
                                        Console.WriteLine("  Input must be an Non negative / whole / numeric value.");
                                        Console.ReadLine();
                                        ProjectData.EndApp();
                                }
                                else
                                {
                                        // if it is not charcter and not fractional then cast  it to integer
                                        value = int.Parse(temp);
                                }
                               
                                //if number is negative
                                if (value < 0)
                                {
                                        Console.WriteLine();
                                        Console.WriteLine("  Input must be an Non negative / whole / numeric value.");
                                        Console.ReadLine();
                                        ProjectData.EndApp();
                                }
                        }
                        catch (InvalidCastException)
                        {
                                Console.WriteLine();
                                Console.WriteLine("  Input must be an Non negative / whole / numeric value.");
                                Console.ReadLine();
                                ProjectData.EndApp();
                        }
                        return value;
                }
        }
       
}