In calculator use string split

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections;


namespace WindowsFormsApplication6
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

private void button1_Click(object sender, EventArgs e)
{
int counter = 0;
string inputText = Input.Text;
Stack stack = new Stack();
char[] chars = inputText.ToCharArray();
string lastNumber = string.Empty;



foreach (char c in chars)
{
if (char.IsDigit(c))
{
lastNumber = lastNumber + c.ToString();
}
else
{

stack.Push(lastNumber);
stack.Push(c);
lastNumber = string.Empty;
}
counter++;
}
stack.Push(lastNumber);


int prevNum, currNum=0;
string operand;
bool isEnd = true;

prevNum = int.Parse(stack.Pop().ToString());
operand = stack.Pop().ToString();


do {

operand = stack.Pop().ToString();
if (operand == "+")
{
currNum = int.Parse(stack.Pop().ToString());

prevNum = currNum + prevNum;
answer.Text = prevNum.ToString();
}

if (operand == "-")
{
currNum = int.Parse(stack.Pop().ToString());
prevNum = currNum - prevNum;
answer.Text = prevNum.ToString();
}

if (operand == "/")
{
currNum = int.Parse(stack.Pop().ToString());
prevNum = currNum / prevNum;
answer.Text = prevNum.ToString();
}

if (operand == "*")
{
currNum = int.Parse(stack.Pop().ToString());
prevNum = currNum * prevNum;
answer.Text = prevNum.ToString();
}
}while (operand!=" ");

}


}
}





I do these coding but have an error int ToString() method . any1 help to solve it.