#[macro_export]
macro_rules! setup {
    { mut $input:ident: SplitWhitespace $(,)? } => {
        use std::io::Read;
        let mut buf = String::new();
        std::io::stdin().read_to_string(&mut buf).unwrap();
        let mut $input = buf.split_whitespace();
    };
}

#[macro_export]
macro_rules! parse_next {
    ($str_iter:expr) => {
        $str_iter.next().unwrap().parse().unwrap()
    };
}

fn main() {
    setup! { mut input: SplitWhitespace };

    let l: usize = parse_next!(input);
    let k: usize = parse_next!(input);

    let ans = match (l as f64) / ((k * 2) as f64) {
        x if x.trunc() == x => ((x.trunc() as usize) - 1) * k,
        x => (x.trunc() as usize) * k,
    };

    println!("{}", ans);
}