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)

#![allow(unused_variables)]
#![allow(unused_imports)]
#![allow(dead_code)]
use std::io;
use std::str::FromStr;
use std::iter::FromIterator;
use std::cmp::{max, min, Ordering};
use std::collections::*;
use std::mem::{replace, swap};


// abcde
fn read_string() -> String {
    let mut line = String::new();
    io::stdin().read_line(&mut line).unwrap();
    line
}

// -3.2
fn read_as_primitive<T: FromStr>() -> T {
    let s = read_string();
    s.trim().parse().ok().expect("failed to parse input")
}

// 1 -3 0 999
fn read_as_collection<U: FromStr, T: FromIterator<U>>() -> T {
    let s = read_string();
    s.trim().split_whitespace().map(|s| s.parse().ok().expect("failed to parse")).collect()
}

// #.#.###..
fn read_as_chars<T: FromIterator<char>>() -> T {
    let s = read_string();
    s.trim().chars().collect()
}

// 1 abc
fn read_as_pair<T: FromStr, U: FromStr>() -> (T, U) {
    let s = read_string();
    let elems: Vec<_> = s.trim().split_whitespace().collect();
    (
        elems[0].parse().ok().expect("failed to parse"),
        elems[1].parse().ok().expect("failed to parse"),
    )
}

// 1.5 -2 abc
fn read_as_triple<T: FromStr, U: FromStr, V: FromStr>() -> (T, U, V) {
    let s = read_string();
    let elems: Vec<_> = s.trim().split_whitespace().collect();
    (
        elems[0].parse().ok().expect("failed to parse"),
        elems[1].parse().ok().expect("failed to parse"),
        elems[2].parse().ok().expect("failed to parse"),
    )
}

fn main() {
    let (ab, bc, ca): (i32, i32, i32) = read_as_triple();
    println!("{}", ab * bc / 2);
}