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).

Examples

Input

3


Output

2


Input

6


Output

9


Input

11


Output

30


Input

50


Output

625
import java.util.Scanner;

public class Main {
	public static Scanner sc = new Scanner(System.in);
	public static void main(String[] args){
		//A
		int K = sc.nextInt();
		int even = K/2;
		int odd = even + (K % 2);
		System.out.println(K*(K-1)/2-even*(even-1)/2-odd*(odd-1)/2);
	}
}
