# Task
Imagine `n` horizontal lines and `m` vertical lines.
Some of these lines intersect, creating rectangles.
How many rectangles are there?
# Examples
For `n=2, m=2,` the result should be `1`.
there is only one 1x1 rectangle.
For `n=2, m=3`, the result should be `3`.
there are two 1x1 rectangles and one 1x2 rectangle. So `2 + 1 = 3`.
For n=3, m=3, the result should be `9`.
there are four 1x1 rectangles, two 1x2 rectangles, two 2x1 rectangles and one 2x2 rectangle. So `4 + 2 + 2 + 1 = 9`.
# Input & Output
- `[input]` integer `n`
Number of horizontal lines.
Constraints: `0 <= n <= 100`
- `[input]` integer `m`
Number of vertical lines.
Constraints: `0 <= m <= 100`
- `[output]` an integer
Number of rectangles.
def rectangles(n, m):
  return n*m*(n-1)*(m-1)//4
def rectangles(n, m):
  return n*m*(n-1)*(m-1)//4