def gcd(num1, num2):
    '''Returns the GCD of num1 and num2.'''
    x = max(num1, num2)
    y = min(num1, num2)
    r = x % y
    while r != 0:
        x = y
        y = r
        r = x % y
    return y


def gcd_interactive():
    '''Repeatedly prompts the user for two numbers, then displays their GCD.
    The user can end the prompts by entering 'quit' for the first number.'''
    
    finished = False
    while not finished:
        num1 = input('Input a number: ')
        if num1 == 'quit':
            finished = True
        else:
            num2 = input('Input a second number: ')
            result = gcd(int(num1), int(num2))
            print('The GCD of {0} and {1} is {2}'.format(num1, num2, result))

    print('all done!')


def file_city_sum():
    '''Sums the population of a number of cities listed in a file.'''

    file = open('population.txt') # open a file object
    total_pop = 0
    for line in file: # loop through the lines of the file
    	parts = line.split()
    	city_name = parts[0]
    	city_pop = int(parts[1])
    	total_pop += city_pop
    file.close()

    print('Total population is', total_pop)