def additaet(n):
    """ sums all numbers from 0 to n using Gaussian formula.
	basically same performance
		(n*n + n) >> 1
	too slow, floats
		int(((n/2.0) + 0.5) * n)
		int((n + 1) / 2.0 * n)
	too slow, worse complexity
		sum(range(n+1))
	"""
    return (n + 1) * n >> 1
