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).

Sample Input 1

3

Sample Output 1

2

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

Sample Input 2

6

Sample Output 2

9

Sample Input 3

11

Sample Output 3

30

Sample Input 4

50

Sample Output 4

625