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 mut current_size = input[0];
    let mut count = 0;
    for _i in 0..max {
        current_size -= input[1];
        if current_size <= input[1] {
            break;
        }
        current_size -= input[1];
        count += 1;
    }
    println!("{}", count * input[1]);
}