def number_sum(n): 
    '''Returns the sum of the first n numbers''' 
    assert(n >= 0), 'n must be >= 0' 
    
    if n == 0:
        return 0
    else:
        return n + number_sum(n-1)  
 
if __name__ == '__main__': 
    from timeit import Timer 
    t = Timer('number_sum(30)', 'from __main__ import number_sum')
    print('Time: ', t.timeit())