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 = 0
  while (0 < L - 2 * K) {
    result += K
    L -= 2 * K
  }
  console.log(result)
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"))