using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCorder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var N = cin.ReadInt;
            var T = cin.ReadInt;
            var t = cin.ReadIntArray(N);
            var EndTime = (long)T;
            var ans = (long)T;
            for(int i = 1; i < N; i++)
            {
                if (t[i] >= EndTime)
                {
                    ans += T;
                }
                else
                {
                    ans += T - (EndTime - t[i]);
                }
                EndTime = t[i] + T;
            }
            WriteLine(ans);
        }



        public long C(int X, int Y)
        {

            if (Y == 0 || Y == X)
            {
                return 1;
            }
            if (X < Y)
            {
                return 0;
            }
            var Pascal = new long[X + 1, X + 1];
            for (int i = 0; i <= X; i++)
            {
                Pascal[i, 0] = 1L;
                Pascal[i, i] = 1L;
            }
            for (int i = 2; i <= X; i++)
            {
                for (int j = 1; j < i; j++)
                {
                    Pascal[i, j] = Pascal[i - 1, j] + Pascal[i - 1, j - 1];
                }
            }
            return Pascal[X, Y];
        }

        public class ConsoleInput
        {
            private readonly System.IO.TextReader _stream;
            private char _separator = ' ';
            private Queue<string> inputStream;
            public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
            {
                this._separator = separator;
                this._stream = stream;
                inputStream = new Queue<string>();
            }
            public string Read
            {
                get
                {
                    if (inputStream.Count != 0) return inputStream.Dequeue();
                    string[] tmp = _stream.ReadLine().Split(_separator);
                    for (int i = 0; i < tmp.Length; ++i)
                        inputStream.Enqueue(tmp[i]);
                    return inputStream.Dequeue();
                }
            }
            public string ReadLine { get { return _stream.ReadLine(); } }
            public int ReadInt { get { return int.Parse(Read); } }
            public long ReadLong { get { return long.Parse(Read); } }
            public double ReadDouble { get { return double.Parse(Read); } }
            public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
            public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
            public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
        }
    }
}


Measured memory_bytes: 32132