function main(input) {
	input = input.split(" ");
	var n = parseInt(input[0],10);
	var d = parseInt(input[1],10);
    var dcount = (d * 2) + 1;
    var kanshi = n / dcount;
 

    console.log(Math.ceil(kanshi))

}
	

main(require('fs').readFileSync('/dev/stdin', 'utf8'));