// includes
#include <cstdio>
#include <cstdint>
#include <iostream>
#include <iomanip>
#include <string>
#include <queue>
#include <stack>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <utility>
#include <functional>
#include <cmath>
#include <climits>
#include <bitset>
#include <list>
#include <random>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())

using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;

// solve
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}

bool comp(const Pl &x, const Pl &y){
  return x.first < y.first;
}

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n;
  ll l;
  cin >> n >> l;
  vector<ll> x(n), y(n);
  rep(i, n)cin >> x[i];
  rep(i, n)cin >> y[i];
  vector<int> d(2 * n, 0);
  int mini = inf, maxi = -1;
  int mi = 0, ma = 0;
  vector<Pl> vec;
  rep(i, n){
    vec.pb(mk(x[i], 1));
    vec.pb(mk(y[i], -1));
  }
  sort(all(vec), comp);
  rep(i, 2 * n){
    d[i] = vec[i].second;
    if(i > 0)d[i] += d[i-1];
    if(maxi <= d[i]){
      maxi = d[i];
      ma = i;
    }
    if(mini > d[i]){
      mini = d[i];
      mi = i;
    }
  }
  ll tmp = 0;
  if(mini < 0){
    tmp = vec[mi].first;
    vector<Pl> v;
    bool flag = true;
    for(int i = mi + 1; i < 2 * n; i++){
      v.pb(mk(vec[i].first - tmp, vec[i].second));
    }
    for(int i = 0; i < mi; i++){
      if(flag && vec[i].second == 1){
        flag = false;
        continue;
      }
      v.pb(mk(vec[i].first - tmp + l, vec[i].second));
    }
    if(sz(v) == 2 * n - 1){
      v.insert(v.end(), mk(l, -1));
    }
    d.assign(sz(v), 0);
    maxi = -1, mini = inf, ma = 0, mi = 0;
    rep(i, sz(v)){
      d[i] = v[i].second;
      if(i > 0)d[i] += d[i-1];
      if(maxi <= d[i]){
        maxi = d[i];
        ma = i;
      }
      if(mini > d[i]){
        mini = d[i];
        mi = i;
      }
    }
    ll res = (maxi - 1) * l + v[ma+1].first;
    cout << res + tmp << endl;
  }else{
    ll res = (maxi - 1) * l + vec[ma+1].first;
    cout << res + tmp << endl;
  }
	return 0;
}