import std.stdio, std.conv, std.string, std.range, std.algorithm, std.array,
  std.functional, std.container, std.typecons;

void main() {
  auto input = readln.split.to!(int[]);
  auto N = input[0], D = input[1];

  auto r = 2*D + 1;
  ((N + r -1) / r).writeln;
}
