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
$k = trim(fgets(STDIN));

$count=0;
for($i=1; $i<=$k; $i++) {
    if($i%2!=0) continue;
    for($j=1; $j<=$k; $j++) {
        if($j%2==0) continue;
        $count++;
    }
}

echo $count;