"use strict"
function main(arg) {
    //とりあえず行を分割
    var input = arg.split(' ');
    var ki = input[0];
    var mieru = input[1];

    var kakeru2 = mieru * 2 ;
    var kakeru = kakeru2 + 1;


    var kotae = Math.ceil(ki / kakeru);


    console.log(kotae);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));