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)

open Printf
open Scanf

let id x = x

let k = scanf "%d" id

let () =
    let n = k / 2 in
    if k mod 2 = 0 then begin
      printf "%d\n" (n * n)
    end else begin
      printf "%d \n" (n * (n + 1))
    end