/*
  +-------------+  
  |             |  
  o---o x o x o x x
      | | | | |   |
      +-+ +-+ +---+
*/

import std.conv, std.functional, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int N;
long L;
long[] X, Y;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      L = readLong();
      X = new long[N];
      Y = new long[N];
      foreach (i; 0 .. N) {
        X[i] = readLong();
      }
      foreach (i; 0 .. N) {
        Y[i] = readLong();
      }
      
      auto xs = X.redBlackTree!true;
      auto ys = Y.redBlackTree!true;
      debug {
        writeln("xs = ", xs);
        writeln("ys = ", ys);
      }
      
      long ans = 0, cur = 0;
      foreach (i; 0 .. N) {
        auto rx = xs.upperBound(cur);
        if (rx.empty) {
          rx = xs[];
        }
        const x = rx.front;
        xs.removeKey(x);
        auto ry = ys.upperBound(x);
        if (ry.empty) {
          ry = ys[];
        }
        long y = ry.front;
        if (!rx.empty) {
          auto rz = xs.upperBound(y);
          if (rz.empty) {
            rz = xs[];
          }
          const z = rz.front;
          auto rw = ys.lowerBound(z);
          if (rw.empty) {
            rw = ys[];
          }
          const w = rw.back;
          y = w;
        }
        ys.removeKey(y);
        debug {
          writefln("o %s, x %s", x, y);
        }
        if (cur > x) {
          ans += L;
        }
        if (x > y) {
          ans += L;
        }
        cur = y;
      }
      ans += cur;
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}