task3

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
// you can also use other imports, for example:
// using System.Collections.Generic;

// you can write to stdout for debugging purposes, e.g.
// Console.WriteLine("this is a debug message");

class Solution {
    static void checkNumberSize(int n)
    {
        if (n < 0 || n > (Math.Pow(2, 20) - 1))
        {
            throw new Exception();
        }
    }

    public int solution(string S) {
        try
        {
            Stack<int> stack = new Stack<int>();
            string[] instructions = S.Split(' ');
            Console.WriteLine();
            for (int i = 0; i < instructions.Length; i++)
            {
                switch (instructions[i])
                {
                    case "+":
                        if (stack.Count < 2)
                        {
                            throw new Exception();
                        }
                        else {
                            int num = stack.Pop() + stack.Pop();
                            checkNumberSize(num);
                            stack.Push(num);
                        }
                        break;
                    case "-":
                        if (stack.Count < 2)
                        {
                            throw new Exception();
                        }else
                        {
                            int second = stack.Pop();
                            int first = stack.Pop();
                            int num = second - first;
                            checkNumberSize(num);
                            stack.Push(num);
                        }
                        break;
                    case "POP":
                        stack.Pop();
                        break;
                    case "DUP":
                        stack.Push(stack.Peek());
                        break;
                    default:
                        int numToPush = Convert.ToInt32(instructions[i]);
                        checkNumberSize(numToPush);
                        stack.Push(numToPush);
                        break;
                }
            }
            return stack.Peek();
        }
        catch (Exception)
        {
            return -1;
        }
    }
}