#include<bits/stdc++.h>

using namespace std;

const int inf = 1 << 30;

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];
    int match_min = inf;
    for(auto &p : mp) {
      if(st[p.second ^ 1].empty()) {
        if(st[0].size() + st[1].size() <= sz) {
          st[p.second].emplace_back(p.first);
        } else {
          return make_pair(false, -1);
        }
      } else {
        st[p.second ^ 1].pop_back();
        st[p.second].emplace_back(p.first);
      }
    }
    if(st[1].empty()) return make_pair(true, L + match_min);
    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;
}