use std::io;
use std::io::{Stdin, BufRead};


struct Input {
    l: u32,
    k: u32,
}


fn main() {
    let mut stdin = io::stdin();
    let input = read_input(&mut stdin);

    let mut pocky_length = input.l;
    let one_bite = input.k;

    let mut bite_count = 0;

    loop {
        if pocky_length <= one_bite * 2 {
            break;
        }
        pocky_length -= one_bite * 2;
        bite_count += 1;
    }

    let yu_bite = bite_count * one_bite;
    println!("{}", yu_bite);
}


fn read_input(stdin: &mut Stdin) -> Input {
    let vec = read_vec();

    Input { l: vec[0], k: vec[1] }
}

fn read_single<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read_single::<String>().split_whitespace()
        .map(|e| e.parse().ok().unwrap()).collect()
}