5.3.1

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            bool isExit = false;
            List<int> numbers = new List<int>();
            while (!isExit)
            {
                string input = Console.ReadLine();
                switch (input)
                {
                    case "exit":
                        isExit = true;
                        break;
                    case "sum":
                        int sum = 0;
                        foreach (var number in numbers)
                        {
                            sum += number;
                        }

                        Console.WriteLine("Сумма всех веденных чисел: " + sum);
                        break;
                    default:
                        int isNumber;
                        bool success = Int32.TryParse(input, out isNumber);

                        if (success)
                        {
                            numbers.Add(Convert.ToInt32(input));
                        }
                        else
                        {
                            Console.WriteLine("Введите число!");
                        }

                        break;
                }
            }
        }
    }
}