Problem:
Score : 100 points

Problem Statement

There is a right triangle ABC with ∠ABC=90°.

Given the lengths of the three sides, |AB|,|BC| and |CA|, find the area of the right triangle ABC.

It is guaranteed that the area of the triangle ABC is an integer.

Constraints

1 \leq |AB|,|BC|,|CA| \leq 100

All values in input are integers.

The area of the triangle ABC is an integer.

Input

Input is given from Standard Input in the following format:

|AB| |BC| |CA|

Output

Print the area of the triangle ABC.

Sample Input 1

3 4 5

Sample Output 1

6

This triangle has an area of 6.

Sample Input 2

5 12 13

Sample Output 2

30

This triangle has an area of 30.

Sample Input 3

45 28 53

Sample Output 3

630

This triangle has an area of 630.

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::io::Read;
use std::collections::BTreeSet;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();

    let mut set = buf.split_whitespace()
        .map(|s| s.parse::<usize>().unwrap())
        .collect::<BTreeSet<usize>>();

    let r = set.clone();
    let max = r.iter().max().unwrap();
    set.remove(max);

    println!("{}", set.iter().product::<usize>() / 2)
}