import java.util.regex.Matcher
import java.util.regex.Pattern

fun main(args: Array<String>) {
    fun readLineLongArray(): List<Long> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toLong() }
        return ret
    }

    fun readLineLong(): Long {
        val str = readLine() as String
        return str.toLong()
    }

    fun readLineInt(): Int {
        val str = readLine() as String
        return str.toInt()
    }

    fun readLineIntArray() : List<Int> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toInt() }
        return ret
    }

    fun readLineDoubleArray(): List<Double> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toDouble() }
        return ret
    }

    fun readLineDouble() : Double {
        val str = readLine() as String
        return str.toDouble()
    }

    fun readLineString() : String {
        return readLine() as String
    }

    //    fun readLineStringArray(): List<String> {
    //        val str = readLine() as String
    //        val arrStr = str.split(" ")
    //        return arrStr
    //    }

    val n = readLineInt()
    if (n == 1) {
        println(0)
        return
    }
    val l = readLineInt()
    var ave = 0.toDouble()
    var aveList = Array(n-1,{0.toDouble()})
    var list = arrayListOf<Int>(0)
    for (i in 1..(n-1)) {
        val tmp = readLineInt()
        val bf = list.last()
        if (Math.abs(tmp-bf) >= l) {
            print(-1)
            return
        }
        list.add(tmp)
//        ave += tmp
    }
    var tmpS = 0.toDouble()
    for (i in (aveList.size) downTo 1) {
//        for (j in (i+1)..(n-1)) {
//            aveList[i] += list[j]
//        }
        tmpS += list[i]
        aveList[i-1] = tmpS
    }
    for (i in 0..(aveList.size-1)) {
        aveList[i] += (l * (n-i-1)).toDouble() / 2.toDouble()
        aveList[i] /= n-i-1
    }
//    ave += (l * n).toDouble() / 2.toDouble()
//    ave /= n
//    println(ave)
//    for (item in aveList) {
//        println(item)
//    }
    var count = 0
    for (i in 0..(n-2)) {
        if (list[i] < aveList[i] && aveList[i] < (list[i]+l)
        && list[i+1] < aveList[i] && aveList[i] < (list[i+1]+l)) {

        } else {
            count++
        }
    }
    print(count)
}