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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php /*atcoder.php*/

$at = new Atcoder();
$at->main();
unset($at);

class Atcoder{
	private function lineIn(){
		$buf = fgets(STDIN);
		return $this->killNewLine($buf);
	}
	private function killNewLine($buf = null){
		$buf = str_replace("\r\n", "", $buf);
		$buf = str_replace("\n", "", $buf);
		$buf = str_replace("\r", "", $buf);
		return $buf;
	}

	public function main(){
		$s = $this->lineIn();
	
		$ans = 0;
		$h = 0;
		$t = 0;
		
		$f = true;
		for($i = 1; $i < $s; $i++){
			if($f){
				$h++;
			}else{
				$t++;
			}
			$f = (!$f);
		}

		$ans = ($h * $t) + floor($s / 2);
		
		echo $ans . "\n";

	}
}
?>