public static void Main()
Console.WriteLine("Please enter your credit card number:");
string crdnum = Console.ReadLine();
for (p = 0; p < crdnum.Length; p++)
digit = crdnum.Substring(p, 1);
newNumber += (Convert.ToInt32(digit) * 2);
for (p = 0; p < newNumber.Length; p++)
sum += Convert.ToInt32(newNumber.Substring(p, 1));
Console.WriteLine("Your card is valid");
Console.WriteLine("Your card is invalid");