Calculator_functions

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

namespace Calculator
{
    internal class Program
    {

        static int AskAboutInteger()
        {
            int val;
            Console.Write($"Enter a number: ");
            val = int.Parse(Console.ReadLine());
            return val; ;
        }

        static int PrintMenu()
        {
            Console.WriteLine("0 - Exit");
            Console.WriteLine("1 - Ask about the numbers");
            Console.WriteLine("2 - Add");
            Console.WriteLine("3 - Sub");
            Console.WriteLine("4 - Multi");
            Console.WriteLine("5 - Div");
            Console.Write("Your choice: ");
            int choice = int.Parse(Console.ReadLine());
            return choice;
        }
        static int Add(int a, int b)
        {
            return a + b;
        }
        static int Sub(int a, int b)
        {
            return a - b;
        }
        static int Multi(int a, int b)
        {
            return a * b;
        }

        static double? Div(int a, int b)
        {
            double? res;
            if (b != 0)
            {
                res = (double)a / b;
                return res;
            }
            else
            {
                Console.Clear();
                Console.WriteLine("\n\n\n\n\n\t\t\t\t\tDon't devide by 0!");
                Console.ReadKey();
                return null;
            }
        }

        static void PrintResult(int a, int b, double? r, char o)
        {
            Console.WriteLine($"\n\n\n\n\n\t\t\t\t\t\t{a} {o} {b} = {r}");
            Console.ReadKey();

        }


        static void Main(string[] args)
        {
            char operation = ' ';
            int decision;
            int num1 = 0, num2 = 0;
            double? result = null;

            do
            {   Console.Clear();
                decision = PrintMenu();
                Console.Clear();
                switch (decision)
                {

                    case 0:
                        Console.WriteLine("\n\n\n\n\n\t\t\t\t\t\tThank you!");
                        result = null;
                        System.Threading.Thread.Sleep(1000);
                        break;
                    case 1:
                        num1 = AskAboutInteger();
                        num2 = AskAboutInteger();
                        result = null;
                        break;
                    case 2:
                        result = Add(num1, num2);
                        operation = '+';
                        break;
                    case 3:
                        result = Sub(num1, num2);
                        operation = '-';
                        break;
                    case 4:
                        result = Multi(num1, num2);
                        operation = '*';
                        break;
                    case 5:
                        result = Div(num1, num2);
                        operation = '/';
                        break;
                }
                if (result != null)
                    PrintResult(num1, num2, result, operation);
            } while (decision != 0);
        }
    }
}