#include <algorithm>
#include <cassert>
#include <cctype>
#include <chrono>
#define _USE_MATH_DEFINES
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <tuple>
#include <vector>
using namespace std;

#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()

const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3fLL;
const int MOD = 1000000007; // 998244353
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
/*-------------------------------------------------*/
int main() {
  cin.tie(0); ios::sync_with_stdio(false);
  // freopen("input.txt", "r", stdin);

  int n, l; cin >> n >> l;
  vector<pair<int, bool> > xy; // true -> sushi
  REP(i, n) {
    int x; cin >> x;
    xy.emplace_back(x, true);
  }
  REP(i, n) {
    int y; cin >> y;
    xy.emplace_back(y, false);
  }
  sort(ALL(xy));
  vector<int> d(2 * n);
  int m = -INF, m_ = INF, p, q;
  REP(i, 2 * n) {
    d[i] = (xy[i].second ? 1 : -1);
    if (i > 0) d[i] += d[i - 1];
    if (d[i] >= m) {
      m = d[i];
      p = i;
    }
    if (d[i] < m_) {
      m_ = d[i];
      q = i;
    }
  }
  long long t = 0;
  if (m_ < 0) {
    int bye = -1;
    REP(i, q) {
      if (xy[i].second) {
        bye = i;
        break;
      }
    }
    vector<pair<int, bool> > tmp(xy);
    xy.clear();
    FOR(i, q + 1, 2 * n) {
      int umg = tmp[i].first - tmp[q + 1].first;
      xy.emplace_back(umg, tmp[i].second);
    }
    REP(i, q) if (i != bye) {
      int umg = l - tmp[q + 1].first + tmp[i].first;
      xy.emplace_back(umg, tmp[i].second);
    }
    if (bye == -1) {
      int umg = l - tmp[q + 1].first + tmp[q].first;
      xy.emplace_back(umg, tmp[q].second);
    } else {
      n -= 2;
    }
    t += tmp[q + 1].first;
    m = -INF;
    REP(i, 2 * n) {
      d[i] = (xy[i].second ? 1 : -1);
      if (i > 0) d[i] += d[i - 1];
      if (d[i] >= m) {
        m = d[i];
        p = i;
      }
    }
  }
  t += (long long)(m - 1) * l + xy[p + 1].first;
  cout << t << '\n';
  return 0;
}