sum_cache = {0:0}
  
def number_sum(n): 
    '''Returns the sum of the first n numbers''' 
    assert(n >= 0), 'n must be >= 0'
    
    if n in sum_cache:
        return sum_cache[n]
    res = n + number_sum(n-1)
    # Add the value to the cache
    sum_cache[n] = res
    return res
         
if __name__ == '__main__': 
    from timeit import Timer 
    t = Timer('number_sum(300)', 'from __main__ import number_sum')
    print('Time: ', t.timeit())