"use strict";
const main = arg => {
  let N = arg.split(" ")[0]*1 + 1;
  let D = arg.split(" ")[1]*1 + 1;
  let count = 0;
  while(N > 0){
  	N = N - 2*D;
    count++;
  }
  console.log(count);
  
}

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