using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.IO.Compression;
using System.Text;

namespace Solver
{
    class Program
    {
        const int M = 1000000007;
        const double eps = 1e-9;
        static int[] dd = { 0, 1, 0, -1, 0 };
        static void Main()
        {
            var sw = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            var sc = new Scan();
            var n = sc.Int;
            var l = sc.Int;
            var x = new int[n];
            for (int i = 1; i < n; i++)
            {
                x[i] = sc.Int;
            }
            double g = x[n - 1];
            int ans = 0;
            for (int i = 2; i <= n; i++)
            {
                if (Math.Abs(x[n - i] - x[n - i + 1]) >= l)
                {
                    Console.WriteLine(-1);
                    return;
                }
                if (Math.Abs((g - x[n - i + 1]) * 2) - l > -eps || Math.Abs((g - x[n - i]) * 2) - l > -eps)
                    ++ans;
                    
                g = (g * (i - 1) + x[n - i]) / i;
            }
            sw.WriteLine(ans);
            sw.Flush();
        }
 
        static void swap<T>(ref T a, ref T b)
        {
            var t = a;
            a = b;
            b = t;
        }
 
        static bool isprime(long n)
        {
            if (n == 1) return false;
 
            for (long i = 2; i * i <= n; i++)
                if (n % i == 0)
                    return false;
 
            return true;
        }
 
        static string strsort(string s)
        {
            var c = s.ToCharArray();
            Array.Sort(c);
            return string.Join("", c);
        }
        static string strswap(string s, int i, int j)
        {
            var c = s.ToCharArray();
            c[i] = s[j];
            c[j] = s[i];
            return string.Join("", c);
        }
 
        static T[] copy<T>(T[] a)
        {
            var ret = new T[a.Length];
            for (int i = 0; i < a.Length; i++) ret[i] = a[i];
            return ret;
        }
    }
    class Scan
    {
        public int Int { get { return int.Parse(Console.ReadLine().Trim()); } }
        public long Long { get { return long.Parse(Console.ReadLine().Trim()); } }
        public string Str { get { return Console.ReadLine().Trim(); } }
        public int[] IntArr { get { return Console.ReadLine().Trim().Split().Select(int.Parse).ToArray(); } }
        public int[] IntArrWithSep(char sep) { return Console.ReadLine().Trim().Split(sep).Select(int.Parse).ToArray(); }
        public long[] LongArr { get { return Console.ReadLine().Trim().Split().Select(long.Parse).ToArray(); } }
        public double[] DoubleArr { get { return Console.ReadLine().Split().Select(double.Parse).ToArray(); } }
        public string[] StrArr { get { return Console.ReadLine().Trim().Split(); } }
        public List<int> IntList { get { return Console.ReadLine().Trim().Split().Select(int.Parse).ToList(); } }
        public List<long> LongList { get { return Console.ReadLine().Trim().Split().Select(long.Parse).ToList(); } }
        public void Multi(out int a, out int b) { var arr = IntArr; a = arr[0]; b = arr[1]; }
        public void Multi(out int a, out int b, out int c) { var arr = IntArr; a = arr[0]; b = arr[1]; c = arr[2]; }
        public void Multi(out int a, out int b, out int c, out int d) { var arr = IntArr; a = arr[0]; b = arr[1]; c = arr[2]; d = arr[3]; }
        public void Multi(out int a, out string b) { var arr = StrArr; a = int.Parse(arr[0]); b = arr[1]; }
        public void Multi(out string a, out int b) { var arr = StrArr; a = arr[0]; b = int.Parse(arr[1]); }
        public void Multi(out int a, out int b, out string c) { var arr = StrArr; a = int.Parse(arr[0]); b = int.Parse(arr[1]); c = arr[2]; }
        public void Multi(out int a, out char b) { var arr = StrArr; a = int.Parse(arr[0]); b = arr[1][0]; }
        public void Multi(out char a, out int b) { var arr = StrArr; a = arr[0][0]; b = int.Parse(arr[1]); }
        public void Multi(out long a, out long b) { var arr = LongArr; a = arr[0]; b = arr[1]; }
        public void Multi(out long a, out int b) { var arr = LongArr; a = arr[0]; b = (int)arr[1]; }
        public void Multi(out int a, out long b) { var arr = LongArr; a = (int)arr[0]; b = arr[1]; }
        public void Multi(out string a, out string b) { var arr = StrArr; a = arr[0]; b = arr[1]; }
    }
    class mymath
    {
        public bool isprime(long a)
        {
            if (a < 2) return false;
            for (long i = 2; i * i <= a; i++)
                if (a % i == 0)
                    return false;
 
            return true;
        }
        public long powmod(long a, long b, long M)
        {
            if (a == 0) return 0;
            if (b == 0) return 1;
            if (b == 1) return a % M;
 
            var t = powmod(a, b / 2, M);
 
            if ((b & 1) == 0) return t * t % M;
            else return t * t % M * a % M;
        }
        public long gcd(long a, long b)
        {
            while (b != 0)
            {
                var t = a % b;
                a = b;
                b = t;
            }
            return a;
        }
        public long lcm(int a, int b) { return a * (long)b / gcd(a, b); }
    }
}