Problem:
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).

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

Solution in Python (3.4.3) (judge verdict: Accepted)

#!/usr/bin/env python3
# -*- coding: utf-8 -*-


def main():
    K = int(input())
    if K % 2 == 0:
        print(int((K / 2) ** 2))
    else:
        print(int(((K - 1) / 2) * int((K + 1) / 2)))


if __name__ == "__main__":
    main()