using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using System.Globalization;
using System.Diagnostics;
using static System.Console;
//using System.Numerics;
//using static System.Math;

class Program
{
    static void Main()
    {
        //SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
        new Program().solve();
        Out.Flush();
    }
    Scanner cin = new Scanner();
    readonly int[] dd = { 0, 1, 0, -1, 0 }; //→↓←↑
    readonly int mod = 1000000007;


    void solve()
    {
        int N = cin.nextint;
        var P = cin.nextlong;
        var H = cin.scanlong;
        long ans = 0;
        var L = new List<long>();

        for (int i = 0; i < N; i++)
        {
            if (i == N - 1 || H[i] > H[i + 1])
            {
                if (L.Count == 0) continue;

                L.Add(H[i]);
                long sum = 0;
                for (int j = 1; j < L.Count; j++)
                {
                    sum += Math.Min(L[j] - L[j - 1], P);
                }

                sum = Math.Min(sum, 2 * P);
                if (i == N - 1) sum = Math.Min(sum, P);
                ans += sum;

                L.Clear();
            }
            else
            {
                L.Add(H[i]);
            }
            //WriteLine(i + " " + ans);
        }

        WriteLine(ans);
    }

}

class Scanner
{
    string[] s; int i;
    char[] cs = new char[] { ' ' };
    public Scanner() { s = new string[0]; i = 0; }
    public string[] scan { get { return ReadLine().Split(); } }
    public int[] scanint { get { return Array.ConvertAll(scan, int.Parse); } }
    public long[] scanlong { get { return Array.ConvertAll(scan, long.Parse); } }
    public double[] scandouble { get { return Array.ConvertAll(scan, double.Parse); } }
    public string next
    {
        get
        {
            if (i < s.Length) return s[i++];
            string st = ReadLine();
            while (st == "") st = ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            i = 0;
            return next;
        }
    }
    public int nextint { get { return int.Parse(next); } }
    public long nextlong { get { return long.Parse(next); } }
    public double nextdouble { get { return double.Parse(next); } }
}