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)
#define repr(i, n) for (int i = (n); i >= 0; --i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define FORR(i, m, n) for (int i = (m); i >= (n); --i)
#define ifaxb(a, x, b) if (a < x && x < b)
#define toInt(x) (x - '0')
#define vi vector<int>
#define vs vector<string>
#define all(x) x.begin(), x.end()
#define INF 2147483647
#define _INF -2147483647
#define pi 3.1415926536
#define MAXN 100004
using namespace std;
typedef long long ll;

int main() {
  int k;
  cin >> k;
  cout << (k / 2) * ((k + 1) / 2) << endl;
  return 0;
}