#include <bits/stdc++.h>
using namespace std;
using i64 = long long;

int main() {
  i64 N,L;
  cin >> N >> L;
  vector<i64> x(N);
  vector<i64> y(N);
  for(int i = 0;i < N;i++) cin >> x[i];
  for(int i = 0;i < N;i++) cin >> y[i];
  vector<pair<i64,i64>> vec;
  for(int i = 0;i < N;i++) vec.push_back({x[i], 1});
  for(int i = 0;i < N;i++) vec.push_back({y[i], -1});
  vector<i64> d(N + 1, 0);
  sort(begin(vec), end(vec));
  i64 MAX = 0;
  i64 MI = 0;
  i64 MIN = 0;
  for(int i = 0;i < N * 2;i++) {
    d[i + 1] = d[i] + vec[i].second;
    if(MAX <= d[i + 1]) {
      MAX = d[i + 1];
      MI = i;
    }
    if(d[i + 1] <= MIN) {
      MIN = d[i + 1];
    }
  }

  i64 delta = MAX - MIN;
  if(MAX == 0) {
    cout << delta * L + y.front();
  }
  else {
    cout << (delta - 1) * L + vec[MI + 1].first;
  }
  cout << endl;
}