Num1 variable does not reset to 0

The num1 variable remains = ans after stopping the current calculation can someone please help me out tried everything. New to C# could be missing something, you don't need to read past here trying to fill space it wont let me submit it without this . This is

static void Main(string[] args)
{
        Boolean x;
        x = true;

        while (x == true)
        {
            double num1;
            double ans;
            double num2;
            string cal;
            Boolean y;
            y = true;

            Console.WriteLine("Enter a number ");
            num1 = Convert.ToInt32(Console.ReadLine());

            while (y == true)
            {
                Console.WriteLine("What calculation do you want to perform ");
                cal = Console.ReadLine();

                Console.WriteLine("Enter a number ");
                num2 = Convert.ToInt32(Console.ReadLine());

                switch (cal)
                {
                    case "+":
                        ans = add(num1, num2);
                        Console.WriteLine("Answer " + ans);
                        num1 = ans;
                        break;

                    case "-":
                        ans = sub(num1, num2);
                        Console.WriteLine("Answer " + ans);
                        num1 = ans;
                        break;

                    case "/":
                        ans = div(num1, num2);
                        Console.WriteLine("Answer " + ans);
                        num1 = ans;
                        break;

                    case "*":
                        ans = mul(num1, num2);
                        Console.WriteLine("Answer " + ans);
                        num1 = ans;
                        break;

                    case "Stop":
                        y = false;
                        break;
                }
            }
        }
    }