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 <string>
#include <utility>
#include <queue>
#include <algorithm>
#include <map>
#include <cmath>
using namespace std;

#define ll long long
#define pb push_back

int main(){

	int K;
	cin>>K;
	int e=0,o=0;
	if(K%2==0){
		e=K/2;
		o=e;
	}else{
		e=(K-1)/2;
		o=e+1;
	}
	cout<<e*o<<endl;

	return 0;
}