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)

#pragma GCC optimize("03")
#include <bits/stdc++.h>
#define ll long long

using namespace std;

int main(){
//	ifstream cin("tst.in");
//	ofstream out("tst.out");
	ios_base::sync_with_stdio(0);
	cin.tie(NULL);
	int n;
	int cnt = 0;
	cin >> n;
	for(int i = 2; i <= n; i += 2)
		for(int j = 1; j <= n; j += 2)
			cnt++;
	cout << cnt;
	return 0;
}