function Main(input) {
	var inputArr = input.split("\n");
	var arr = inputArr[0].split(" ");
 	var n = parseInt(arr[0]);
  	var d = parseInt(arr[1]);
 
  　var ans = Math.floor(n / (d*2 + 1));
　　var sur = n % (d*2 + 1);
  	if (sur > 0) {
      ans++;
    }

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