function Main(input) {
  // const data = input.split("\n");
  let data = input.split(" ");
  let L = data[0];
  let K = data[1];

  let N = Math.floor(L/K);
  let countYu = Math.floor(N/2);

  if (countYu === 1) {
    return 0;
  } else {
    return countYu * K;
  }
  console.log('a');

}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));