CheckAccountNumber - checks checksum for Russian bank account number

gistfile1.cs

private static bool CheckAccountNumber(string bankNumber, string accountNumber)
{
    int[] mask = new[] { 7, 1, 3, 7, 1, 3, 7, 1, 3, 7, 1, 3, 7, 1, 3, 7, 1, 3, 7, 1, 3, 7, 1 };

    if (bankNumber.Length < 3)
    {
        Console.WriteLine("Incorrect bank number.");
        return false;
    }

    if (accountNumber.Length != 20)
    {
        Console.WriteLine("Incorrect account number.");
        return false;
    }

    string checkNumber = bankNumber.Substring(bankNumber.Length - 3, 3) + accountNumber;

    int summ = 0;

    for (int i = 0; i < checkNumber.Length; i++)
    {
        int digitNumber = int.Parse(checkNumber.Substring(i, 1), CultureInfo.InvariantCulture);
        int digitMask = mask[i];

        summ += digitNumber * digitMask % 10;
    }

    return summ % 10 == 0;
}