fn _read() -> (isize, isize) {
    let mut _s = String::new();
    std::io::stdin().read_line(&mut _s).ok();
    let mut _v: Vec<isize> = _s
        .split_ascii_whitespace()
        .map(|e| e.trim().parse().ok().unwrap())
        .collect();
    (_v[0], _v[1])
}

fn main() {
    let (_l, _k) = _read();
    let (l, k) = (_l as f64, _k as f64);

    println!("{}", ((l - 1.0) / (k * 2.0)).floor() * k);
}