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 (Clang 3.8.0) (judge verdict: Accepted)

#include<iostream>
using namespace std;
int main(){
	int a;
	cin>>a;
	if(a%2==0){cout<<a*a/4<<endl;}
	else{cout<<(a/2)*((a+1)/2)<<endl;}
	return 0;
}