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

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, l) = (c[0], c[1]);
        var x = NList;
        var y = NList;
        var len = new List<int>(n * 2 + 1);
        var dep = new List<int>(n * 2 + 1);
        len.Add(0);
        dep.Add(0);
        var xi = 0;
        var yi = 0;
        while (xi < n || yi < n)
        {
            if (yi == n)
            {
                len.Add(x[xi]);
                dep.Add(dep[^1] + 1);
                ++xi;
            }
            else if (xi == n)
            {
                len.Add(y[yi]);
                dep.Add(dep[^1] - 1);
                ++yi;
            }
            else if (x[xi] < y[yi])
            {
                len.Add(x[xi]);
                dep.Add(dep[^1] + 1);
                ++xi;
            }
            else
            {
                len.Add(y[yi]);
                dep.Add(dep[^1] - 1);
                ++yi;
            }
        }
        var lastht = int.MinValue;
        var lasttea = -1;
        for (var i = 1; i < dep.Count; ++i)
        {
            if (dep[i - 1] > dep[i] && lastht <= dep[i])
            {
                lastht = dep[i];
                lasttea = i;
            }
        }
        var max = dep.Max();
        var min = dep.Min();
        if (max == 0) WriteLine((long)l * (max - min) + len[1]);
        else WriteLine((long)l * (max - min - 1) + len[lasttea]);
    }
}