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: Wrong Answer)

#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#define debug cout<<"****************"<<endl
using namespace std;
typedef long long ll;
inline void ju() {
	freopen("***.in","r",stdin);
	freopen("***.out","w",stdout);
}
inline ll rd() {
	ll x=0,f=1; char c=getchar();
	while((c<'0'||c>'9')&&(c!='-')) c=getchar();
	if(c=='-') f=-1,c=getchar();
	while(c>='0'&&c<='9') {
		x=(x<<3)+(x<<1)+c-'0';
		c=getchar();
	}
	return x*f;
}
int main() {
	int n=rd();
	printf("%d\n",n/2*(n+1)/2); 
	return 0;
}