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 <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

#define SORT(c) sort((c).begin(),(c).end())
#define ANS(ans) cout<<(ans)<<endl;
typedef vector<int> VI;


//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<=(b);++i)
#define REP(i,n)  for (int i = 0; i < (n); ++i)


int main() {
	int n;
	cin >> n;

	int odd, even;

	if (n % 2 == 0) {
		odd = n / 2;
		even = n / 2;
	}
	else {
		odd = n / 2 + 1;
		even = n / 2;
	}

	ANS(odd*even);



	return 0;
}