use std::io::{self, Read};

fn read_stdin() -> Vec<String> {
    let mut buffer = String::new();
    io::stdin().read_to_string(&mut buffer).ok();
    buffer.trim().split('\n').map(|s| s.to_string()).collect()
}

fn main() {
    let input = read_stdin()[0].split(' ').map(|s| s.parse::<usize>().unwrap()).collect::<Vec<usize>>();
    let l = *&input[0];
    let k = *&input[1];

    if (l / 2) % k == 0 {
        println!("{}", (l / 2 / k - 1) * k);
    } else {
        println!("{}", l / 2 / k * k);
    }
}