Test.cs

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

namespace ConsoleApplication26
{
    class Program
    {
        static Boolean isNum(String sNum)
        {
            try
            {
                int num = Int32.Parse(sNum);
            }
            catch
            {
                return false;
            }

            return true;
        }

        static int getNum(String Msg)
        {
            String Str;
            do
            {
                Console.Write(Msg);
                Str = Console.ReadLine();
                if (!isNum(Str))
                    Console.WriteLine("Неверный формат");
            } while (!isNum(Str));

            return Convert.ToInt32(Str);
        }

        static void Main(string[] args)
        {
            int a = getNum("Введите A: ");
            int b = getNum("Введите B: ");

            Console.WriteLine("A + B = "+(a+b));
            Console.ReadLine();
        }
    }
}