# summing up all numbers from 0 to N, using a for loop

import string #importing some useful string methods

"""
check_integer takes a string as input and:
- returns True if the string is a valid representation of an integer
- returns False otherwise
"""
def check_integer(my_string):
    # first check: the only legal characters are the minus sign and the digits.
    for character in my_string:
        if not(character in "-0123456789"):
            return False

    # second check: must have at least one digit.
    if (my_string == "") or (my_string == "-"):
        return False

    # third check: the minus sign is only allowed in the beginning.
    if ('-' in my_string[1:]):
        return False

    #if we did not find any violations, we have a valid integer.
    return True



# start of main code
# get N from the user, keep asking until an integer is entered
while True:
    my_string = input("please enter N: ")
    if (check_integer(my_string) == False):
        print("string", my_string, "is not a valid integer")
    else:
        break

N = int(my_string)

# compute the sum
total = 0
for i in range(0, N+1):
    total = total + i

# print the result
print("total =", total)