"""
This program:
- asks the user to enter a string.
- verifies that the string represents a positive integer, and has the format:
- [optional] white space at the start
- [optional] minus sign
- a sequence of consecutive characters of digits, from 0 to 9.
- [optional] white space at the end
"""

text = input("enter an integer: ")
print("checking if", text, "is 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.
if (found_violation == False):
    print("True")
    number = int(text)