function Main(input) {
  input = input.split(' ')
  var N = parseInt(input[0])
  var D = parseInt(input[1])
  var i = 1

  while (N > (2 * D + 1) * i) {
    i++
  }

  console.log(i)
}

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

