function Main(input){
  var In = input.split(' ');
  var N = parseInt(In[0], 10);
  var D = parseInt(In[1], 10);
  var count = 0;
  for(var i = 0 ;i<=N; i += 2*D + 1){
    if(i == N){
      break;
    }
    count += 1;
  }
  
  console.log(count);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));