import math._

object Main {
  def isInvalid(L: Int, xs: Array[Int]): Boolean = {
    var l = 0
    for (i <- 0 to xs.length-1) {
      if (abs(l - xs(i)) >= L) return true
      l = xs(i)
    }
    false
  }

  def calc(N: Int, L: Int, xs: Array[Int]): Int = {
    if (isInvalid(L, xs)) return -1

    if (xs.isEmpty) return 0

    var ans = 0
    var p = (xs.last + xs.last + L) / 2.0
    var cnt = 1
    for (i <- xs.length-2 to 0 by -1) {

      var t = p / cnt
      if (xs(i) < t && t < xs(i) + L && xs(i+1) < t && t < xs(i+1) + L) {
      }
      else ans += 1

      p += (xs(i) + xs(i) + L) / 2.0
      cnt += 1
    }
    ans
  }

  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)

    val N = sc.nextInt
    val L = sc.nextInt
    val xs = Array.fill(N-1)(sc.nextInt)
    println(calc(N, L, Array(0) ++ xs))
  }
}