def digit_sum(n):
    result = 0
    while n > 0:
        divided = n / 10
        whole = int(divided)
        decimal = divided - whole
        result += decimal * 10
        n = whole
    return result


# TESTS

# ultra simple
#print(digit_sum(0))
#print(digit_sum(247))
#print(digit_sum(123456789))
#print(digit_sum(1000000000000000000001))
#print(digit_sum(00000000000000000000000))



def test_digit_sum(n, result):
    print("n = ", n)
    if digit_sum(n) != result:
        print("FAIL")
        print(result, " expected, got ", digit_sum(n))
    else:
        print("PASS")
    print()


if __name__ == "__main__":
    test_digit_sum(0, 0)
    test_digit_sum(247, 13)
    test_digit_sum(123456789, 45)
    test_digit_sum(1000000000000000000000000001, 2)
    test_digit_sum(0000000000000000000000000000000000000, 0)