fn main() {
    let s = {
        let mut s = String::new();
        std::io::stdin().read_line(&mut s).unwrap();
        s.trim_right().to_owned()
    };
    
    let (l, k) = {
        let mut ws = s.split_whitespace();
        let l: u32 = ws.next().unwrap().parse().unwrap();
        let k: u32 = ws.next().unwrap().parse().unwrap();
        (l, k)
    };
    
    if l % (2*k) == 0 {
        println!("{}", l/2 - k);
    }
    else {
        println!("{}", k*(l/(2*k)));
    }
}