fn main() {
    let mut input = String::new();
    let _ = std::io::stdin().read_line(&mut input);
    let input: Vec<i32> = input.trim().split_whitespace().map(|n| n.parse().unwrap()).collect();
    let max = input[0] / (input[1] * 2);
    let rest = input[0] % (input[1] * 2);
    let result = if rest == 0 {
        (max - 1) * input[1]
    } else {
        max * input[1]
    };
    println!("{}", result);
}