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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
typedef long long LL;
using namespace std;

LL V(int M) {
	LL ans = 1;
	for (int pos = 2; pos <= M; pos++)
		ans *= pos;
	return ans;
}

LL C(int M, int N) {
	return V(M) / V(N) / V(N - M);
}

int main() {
	int num; 
	cin >> num;
	cout << (num / 2 * (num - num / 2));
}