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 K;
vector<int>even,odd;
vector<pair<int,int>>pairs;
main(){
  cin>>K;
  for(int i=1;i<=K;++i){
    if(i%2)odd.push_back(i);
    else even.push_back(i);
  }
  for(auto&i:odd)for(auto&j:even)pairs.push_back({i,j});
  cout<<pairs.size()<<endl;
}