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>
#define rep(i,n) for(int i=0; i<(n); i++) 
using namespace std;
typedef long long ll;

int main(){
	int k; cin>>k;
	int cnt=0;
	for(int i=1; i<=k; i++){
		if(i%2==0){ //Even
			for(int j=i+1; j<=k; j++){
				if(j%2!=0) cnt++;
			}
		} else {
			for(int j=i+1; j<=k; j++){
				if(j%2==0) cnt++;
			}
		}
	}
	cout << cnt << endl;	
}