#include<bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
const int inf = (1 << 30) - 1;
const int64 infll = (1LL << 61) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename T >
struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;


int main() {
  int N, L;
  cin >> N >> L;
  vector< pair< int, int > > mp;
  for(int z = 0; z < 2; z++) {
    for(int i = 0; i < N; i++) {
      int p;
      cin >> p;
      mp.emplace_back(p, z);
    }
  }
  sort(begin(mp), end(mp));


  auto check = [&](int sz) {
    vector< int > st[2];
    for(auto &p : mp) {
      if(p.second == 0) {
        if(st[1].empty()) {
          if(st[0].size() + st[1].size() <= sz) {
            st[p.second].emplace_back(p.first);
            continue;
          }
          return make_pair(false, -1);
        }
        st[1].pop_back();
        st[0].emplace_back(p.first);
      } else {
        if(st[0].empty()) {
          if(st[0].size() + st[1].size() <= sz) {
            st[p.second].emplace_back(p.first);
            continue;
          }
          return make_pair(false, -1);
        }
        st[0].pop_back();
        st[1].emplace_back(p.first);
      }
    }
    if(st[1].empty()) throw 0;
    return make_pair(true, st[1][0]);
  };
  int ok = N, ng = -1;
  while(ok - ng > 1) {
    int mid = (ok + ng) / 2;
    if(check(mid).first) ok = mid;
    else ng = mid;
  }
  cout << 1LL * ok * L + check(ok).second << endl;
}