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

import number_check

# get N from the user, keep asking until an integer is entered
while True:
    my_string = raw_input("please enter N: ")
    if (number_check.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