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 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>

#include <iostream>
//#include <algorithm>
// #include <iomanip>
#define ll long long
#define map unordered_map
#define set unordered_set
#define l_l pair<ll, ll>
#define vll vector<ll>
#define mll map<ll, ll>

using namespace std;

const ll MOD = 1000000007LL;
const ll INF = (1LL << 60LL);

int main() {
  // std::cout << std::fixed << std::setprecision(10);

  ll K;
  scanf("%lld", &K);

  ll odd = 1 + (K - 1) / 2;
  ll even = K / 2;

  ll ans = odd * even;

  cout << ans << endl;
}