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 Rust (1.15.1) (judge verdict: Accepted)

use std::io;
use std::io::BufRead;
use std::io::BufReader;

fn main() {
    let reader = io::stdin();
    let mut reader = BufReader::new(reader.lock());

    let mut s = String::new();

    let _ = reader.read_line(&mut s);

    let k: u32 = s.trim().parse().unwrap();

    let tmp = k / 2;

    println!("{}", if k % 2 == 0 { tmp * tmp } else { tmp * (tmp + 1) });
}