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

fn main(){
        let ls: Vec<usize> = getline().trim().split(' ').map(|x| x.parse().unwrap()).collect();

        println!("{}", (ls[0] - 1) / (ls[1] * 2) * ls[1]);
}