Consider a pyramid made up of blocks. Each layer of the pyramid is a rectangle of blocks, and the dimensions of these rectangles increment as you descend the pyramid. So, if a layer is a `3x6` rectangle of blocks, then the next layer will be a `4x7` rectangle of blocks. A `1x10` layer will be on top of a `2x11` layer on top of a `3x12` layer, and so on.

## Task

Given the dimensions of a pyramid's topmost layer `w,l`, and its height `h` (aka the number of layers), return the total number of blocks in the pyramid.

## Examples

`num_blocks(1, 1, 2)` will return `5`. This pyramid starts with a `1x1` layer and has 2 layers total. So, there is 1 block in the first layer, and `2x2=4` blocks in the second. Thus, 5 is the total number of blocks.

`num_blocks(2, 4, 3)` will return `47`. This pyramid has 3 layers: `2x4`, `3x5`, and `4x6`. So, there are `47` blocks total.

## Notes

All parameters will always be postive nonzero integers.

Efficiency is important. There will be:

* 100 'small' cases with `w`, `l`, and `h` below `20`.
* 100 'big' cases with `w`, `l`, and `h` between `1e9` and `1e10`.

Difficulty: EASY
Starter code: def num_blocks(w, l, h):
	

Solutions:
def num_blocks(w, l, h):
	return w * l * h + (w + l) * h * (h - 1) / 2 + h * (h - 1) * (2 * h - 1) / 6


def num_blocks(width, length, height):

	def linsum(n):
		return n * (n + 1) // 2

	def sqrsum(n):
		return n * (n + 1) * (2 * n + 1) // 6
	cube = width * length * height
	stairs = (width + length) * linsum(height - 1)
	corner = sqrsum(height - 1)
	return cube + stairs + corner


def num_blocks(w, l, h):
	return w * l * h + (h - 1) * h * (w + l + 1) // 2 + (h - 2) * (h - 1) * h // 3


from fractions import Fraction

def num_blocks(w, l, h):
	a = h * w * l
	b = Fraction((h - 1) * h * (w + l), 2)
	print(b)
	c = Fraction((h - 1) * h * (2 * h - 1), 6)
	print(c)
	return int(a + b + c)


def num_blocks(w, l, h):
	res = w * l * h
	h -= 1
	A = h * (h + 1) // 2
	B = (h + 1) * (2 * h + 1) * h // 6
	res += (w + l) * A + B
	return res


num_blocks = lambda w, l, h: (2 * h * h + 3 * h * (l + w - 1) + 3 * (2 * l * w - l - w) + 1) * h // 6