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).