Stack-Expressions

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

namespace DataStructures
{
    class Program
    {

        static int Evaluate(string expression)
        {
            Stack<int> nums = new Stack<int>();
            Stack<char> operations = new Stack<char>();

            foreach (var item in expression)
            {
                if (char.IsDigit(item))
                {
                    nums.Push(item - '0');
                }
                else if (item == '(')
                {
                    nums.Push('(');
                }
                else if (item == '+' || item == '*')
                {
                    operations.Push(item);
                }
                else if (item == ')')
                {
                    char op = operations.Pop();
                    if (op == '+')
                    {
                        int result = 0;
                        while (nums.Peek() != '(')
                        {
                            result += nums.Pop();
                        }
                        nums.Pop(); // remove '('
                        nums.Push(result);
                    }
                    else if (op == '*')
                    {
                        int result = 1;
                        while (nums.Peek() != '(')
                        {
                            result *= nums.Pop();
                        }
                        nums.Pop(); // remove '('
                        nums.Push(result);
                    }
                }
            }
            return nums.Peek();

        }

        static int f(int x)
        {
            return 5 * x + 1;
        }

        static int m(int x, int y)
        {
            return (x < y) ? x : y;
        }

        static int s(int x, int y)
        {
            return x - y;
        }

        static int EvalFunction(string expression)
        {
            Stack<int> nums = new Stack<int>();
            Stack<char> func = new Stack<char>();

            foreach (var item in expression)
            {
                if (char.IsDigit(item))
                {
                    nums.Push(item - '0');
                }
                else if (item == 'f' || item == 'm' || item == 's')
                {
                    func.Push(item);
                }
                else if (item == ')')
                {
                    int function = func.Pop();
                    if (function == 'f')
                    {
                        int x = nums.Pop();
                        nums.Push(f(x));
                    }
                    else if (function == 'm')
                    {
                        int y = nums.Pop();
                        int x = nums.Pop();
                        nums.Push(m(x, y));
                    }
                    else if (function == 's')
                    {
                        int y = nums.Pop();
                        int x = nums.Pop();
                        nums.Push(s(x, y));
                    }
                }
            }
            return nums.Peek();
        }

        static void Main(string[] args)
        {
            Console.Write("Enter expression: ");
            string expr = Console.ReadLine();
            Console.WriteLine("{0} = {1}", expr, EvalFunction(expr));
        }

    }
}