function Main(input){
  input = input.split("\n");
  inputStr = input[0].split(" ");
  var N = Number.parseInt(inputStr[0]);
  var D = Number.parseInt(inputStr[1]);
  //結果表示
  console.log(Math.round( N / ( 2*D) ));
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
