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

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] usernames = { "user1", "user2", "user3" };
            string[] passwords = { "1111", "2222", "3333" };

            // data variables
            string username = "john";
            string password = "doe";
            // keyboard input
            string user = "";
            string pass = "";

            do {

                Console.WriteLine("Please enter username:");
                user = Console.ReadLine();
                Console.WriteLine("Please enter password:");
                pass = Console.ReadLine();

                for (int i = 0; i < usernames.Length; i++)
                {
                    if (user == usernames[i])
                    {

                        if (pass == passwords[i])
                        {
                            Console.WriteLine("Welcome to our system!");
                            username = usernames[i];
                            password = passwords[i];
                        }
                        else
                        {
                            Console.WriteLine("Invalid password");
                        }

                    }
                }

            } while (username != user || password != pass);

            Console.ReadKey();
        }
    }
}