use std::io::*;
fn main() {
  let s = stdin();
  let mut buf = String::new();
  s.read_line(&mut buf).ok();
  let l: Vec<u32> = buf
    .trim()
    .split_whitespace()
    .map(|e| e.parse().ok().unwrap())
    .collect();
  let (l, k) = (l[0], l[1]);
  println!("{}",(l-1)/(k*2)*k);
}