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 OCaml (4.02.3) (judge verdict: Accepted)

let f n = match n with
n when (mod) n 2 = 0 -> (n/2)*(n/2)
| n -> (((n-1)/2)+1)*((n-1)/2);;
let () = Scanf.scanf "%d" f
|> Printf.printf "%d\n"