gistfile1.txt

def my_int(x):
    if len(x) == 0:
        return -1

    res = 0
    exp = len(x) - 1

    for c in x:
        digit = ord(c) - ord('0')

        if digit < 0 or digit > 9:
            return -1
        else:
            res += digit * (10 ** exp)
            exp -= 1

    return res

num = input("-> ")
print(my_int(num))