Score : $100$ points

Problem Statement

Find the number of ways to choose a pair of an even number and an odd number from the positive integers between $1$ and $K$ (inclusive). The order does not matter.

Constraints

• $2\leq K\leq 100$
• $K$ is an integer.

Input

Input is given from Standard Input in the following format:

$K$

Output

Print the number of ways to choose a pair of an even number and an odd number from the positive integers between $1$ and $K$ (inclusive).

3

2

Two pairs can be chosen: $(2,1)$ and $(2,3)$.

6

9

11

30

50

625