GS1 Check Digit Calculator

I wrote a check digit calculator, maybe somebody will need it.

public static class CheckDigitCalculator {
    
    public static int CalculateCheckDigit(string input) {
        int sum = CalculateSum(input);
        int checkDigit = CalculateCheckDigit(sum);
        return checkDigit;
    }

    private static int CalculateSum(string input) {
        int factor = 3;
        int sum = 0;
        for (int i = input.Length; i > 0; i--) {
            sum += Convert.ToInt32(input[i - 1].ToString())
                   * factor;
            factor = 4 - factor;
        }
        return sum;
    }

    private static int CalculateCheckDigit(int sum) {
        if (sum % 10 == 0) {
            return 0;
        } else {
            int nearestEqualOrHigherMultipleOfTen =
                (sum / 10) * 10 + 10;
            return nearestEqualOrHigherMultipleOfTen - sum;
        }
    }

}

Note: The perfect one should check for the input string only contains numeric values.