process.stdin.resume();
process.stdin.setEncoding('utf8');
// 自分の得意な言語で
// Let's チャレンジ！！

var lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});
reader.on('line', (line) => {
  lines.push(line);
});
reader.on('close', () => {
 var tree = parseInt(lines[0].split(' ')[0],10);
 var range =parseInt(lines[0].split(' ')[1],10);

 var kansi = 1+2*range;
 var count = 0;
  while(tree>=0){

    count++;
  	tree = tree- kansi;
   if(tree<=0){
   	break;
   }

  }
   console.log(count);


  
});