how to calculate MTBF Calculation

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace MTBF
{
    class Program
    {
        static void Main(string[] args)
        {
            int quantity, quantity_sum = 0;
            int i=1;
            string ch="";
            while (i <= 10)
            {
                Console.WriteLine();
                Console.Write("\nEnter Quantity = ");
                quantity = int.Parse(Console.ReadLine());
                quantity_sum = quantity_sum + quantity;
                Console.Write("\nIs there a breakdown(y or n) = ");
                ch = Console.ReadLine();
                if (ch.Equals("y"))
                {
                    Console.Write("\nBreakdown = " + quantity_sum);
                    quantity_sum = 0;
                }
                else
                {
                //loop will go again to take quantity and add production
                }
                i++;

            }
        }
    }
}