function Main(input) {
  const data = input.trim().split("\n")
  const first_line = data[0].split(" ")
  let L = parseInt(first_line[0]), K = parseInt(first_line[1])
  let result = Math.floor(L / (2 * K)) * K
  if (L - result * 2 === 0) result -= K
  console.log(result)
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"))