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++ (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <queue>
#include <algorithm>
using namespace std;
typedef unsigned int uint;
typedef long long ll;
typedef pair<int, int> P;
const int c_INF = 111111111;
const int c_mINF = -111111111;
const int c_YET = -1;
const int c_Dx[4] = { 0,0,1,-1 };
const int c_Dy[4] = { 1,-1,0,0 };
const int c_div = 1000000007; //10^9 + 7


int main() {

	int K;
	scanf("%d", &K);

	if (K % 2 == 0) {
		printf("%d", K*K / 4);
	}
	else {
		printf("%d", (K - 1) / 2 + (K + 1) / 2);
	}



}