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>
using namespace std;

int main() {
  int K;
  cin >> K;
  
  int count = 0;
  for( int i = 1; i <= K; i++ ){
    for( int j = i+1; j <= K; j++ ){
      if( i % 2 == 0 && j % 2 == 1 ){
        count++;
      }else if( i % 2 == 1 && j % 2 == 0 ){
        count++;
      }
    }
  }
  cout << count << endl;
}