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(); } }