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

namespace CreditCards
{
    class Program
    {
        static bool CreditCardValid(string CreditCardNum)
        {
            int len = CreditCardNum.Length;
            if (CreditCardNum[0] < '3' || CreditCardNum[0] > '7'
                || len > 16 || len < 13)
                return false;
            int sum = 0;

            for (int i = 1, num = 0; i < len; i += 2)
            {
                if (!Char.IsDigit(CreditCardNum[len - i - 1]))
                    return false;
                num = CreditCardNum[len - i - 1] - '0';
                if (num * 2 >= 10)
                {
                    sum += (num * 2) % 10 + (num * 2) / 10;
                }
                else
                    sum += num * 2;
            }
            for (int i = 0; i < len; i += 2)
            {
                sum += CreditCardNum[len - i - 1] - '0';
            }
            if (sum % 10 == 0)
                return true;
            return false;
        }
        static void Main(string[] args)
        {
            if (CreditCardValid("37976208600299"))
            {
                Console.WriteLine("valid card!");
            }
            else
                Console.WriteLine("invalid card!");


        }
    }
}