fn getline() -> String{
    let mut ret = String::new();
    std::io::stdin().read_line(&mut ret).ok();
    ret
}

fn getvec(string: String) -> Vec<i32> {
    string.trim().split_whitespace()
        .map(|s| s.parse().unwrap())
        .collect()
}

fn main() {
    let v = getvec(getline());
    let l = v[0];
    let k = v[1];
    let a =
        if l%(k*2) == 0 {
            (l/k/2-1)*k
        } else {
            l/k/2*k
        };
    println!("{}",a);
}