function main(input) {
	var inputSeparate = input.split(" ");
  	var N = inputSeparate[0];
  	var D = inputSeparate[1];
	var numOfPeople = Math.ceil(N/(D*2+1));
  	console.log(numOfPeople);
  
  	
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));