use std::io::*;
use std::str::*;

const MOD: i32 = 1000000007;

fn read<T: FromStr>() -> Option<T> {
    let stdin = stdin();
    let s = stdin
        .bytes()
        .map(|c| c.unwrap() as char)
        .take_while(|c| !c.is_whitespace())
        .collect::<String>();
    s.parse::<T>().ok()
}

fn main() {
    let mut l: i64 = read().unwrap();
    let k: i64 = read().unwrap();

    let mut ans = 0;
    loop {
        if l - k * 2 <= 0 {
            break;
        }
        l -= k * 2;
        ans += 1;
    }
    println!("{}", ans * k);
}