Parser/recursion problem

[carstenht]

I have just written this small calculator, and it kinda works.
2+2 = 4
4+4 = 8
but
2+2+2 = 12
4-2 = -8

So there is obviosly some glitches, and i suspect the "recurrsion". I have tried lots of adjustments but none solves the problem. Maybe you see something that i dont:

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


namespace minicalc
{
    class Program
    {
        public static String input, tmpexp;       // Expression
        public static int length;         // Lenght of input
        public static int curPos;         // Current token
        public static int value, temp = 0;    // value holders
        public static String Op;

        static void Main(string[] args)
        {



            System.Console.WriteLine("Please enter expression to calculate: ");
            input = System.Console.ReadLine();


            length = input.Length;
            curPos = 0;
            
            while (curPos < length)
            {

                if (input[curPos] == '0' || input[curPos] == '1' || input[curPos] == '2' || input[curPos] == '3' || input[curPos] == '4' || input[curPos] == '5' || input[curPos] == '6' || input[curPos] == '7' || input[curPos] == '8' || input[curPos] == '9')
                {

                    
                    tmpexp += input[curPos];
                    
                    

                }

                
                temp += Convert.ToInt32(tmpexp);
                

                
                Op = Convert.ToString(input[curPos ]);
                

                // Determine which operator we have and use it on the result
                switch (Op)
                {
                    case "+": value += temp;
                        break;
                    case "-": value -= temp;
                        break;
                    case "*": value *= temp;
                        break;
                }

                curPos++;
                


            }

            System.Console.WriteLine("Result of calculation: " + value);
            System.Console.ReadLine();

        }
    }
}