"""
A simple program, that gets an integer from the user,
and prints the result of multiplying that integer by 2.
If the user did not enter a valid integer, the system keeps asking.
"""

while True:
    text = input("please enter an integer: ")
    text = text.strip()

    # found_violation will be set to True the moment we find a violation
    # of the integer format.
    found_violation = False

    # first check: the only legal characters are the minus sign and the digits.
    for character in text:
        if not(character in "-0123456789"):
            found_violation = True
            print("found non-digit character:", character);
            break

    # second check: must have at least one digit.
    if (text == "") or (text == "-"):
        found_violation = True
        print("This string does not contain any digits.")

    # third check: the minus sign is only allowed in the beginning.
    if ('-' in text[1:]):
        found_violation = True
        print("The minus sign is only allowed at the beginning.")

    # If we did not find any violations, we have a valid integer.
    # Now we can exit the while-loop and do whatever we want to do
    # with that integer.
    if (found_violation == False):
        break
        
number = int(text)
print(number * 2)