function main(input){
  var a = input.split(" ");
  var b = a[0];
  var c = a[1];
  var count = 0;
  while(1) {
    b = b - ( c * 2 + 1 );
    count++;
    if (b <= 0) {
      break;
    }
  }
  console.log(count);
}
 
main(require('fs').readFileSync('/dev/stdin', 'utf8'));