import java.io.*;
import java.util.stream.*;

class Process {
    private int L;
    private int K;

    Process(int L, int K) {
        this.L = L;
        this.K = K;
    }

    long getResult() {
        if(L % (2 * K) == 0) {
            return ((long)K * (L / (2 * K) - 1));
        }

        return ((long)K * (L / (2 * K)));
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        var bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        var printWriter = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

        // 入力
        int[] input = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();

        // 処理および出力
        printWriter.println((new Process(input[0], input[1])).getResult());
        
        bufferedReader.close();
        printWriter.close();
    }
}