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 (N <= 2) {  
    console.log(0);
  } else {
    console.log(countYu * K);
  }

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