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 <iostream>
#include <vector>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <limits>
#include <unordered_map>

using namespace std;
using str = string;

#define rep(a,l,r) for(int a=(l);a<(r);a++)
#define get cin>>
#define put cout<<

void run() {

	int k; get k;
	int ans{0};

	if (k%2==1){
		ans = (k / 2) * (k / 2 + 1);
	} else {
		ans = (k / 2) * (k / 2);
	}

	put ans;
}

int main() {ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);run();return 0;}