function main(input) {
    const args = input.split('\n')
    const tmp = args[1].split(' ');
    const n = parseInt(tmp[0], 10);
    const d = parseInt(tmp[1], 10);
    console.log(Math.ceil(n / (d*2+1)))
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));