#include <bits/stdc++.h>
using namespace std;
#define REP(i,N) for(i=0;i<N;i++)
typedef long long ll;
typedef pair<int, int> P;
typedef struct{
  int first;
  int second;
  int third;
}T;

//昇順
bool comp_Se(T& l, T& r){
  return l.second < r.second;
}

int main(void){
  int N;
  ll L;
  cin >> N >> L;
  vector<pair<ll,int> > v(2*N);
  int i;
  REP(i,N){
    cin >> v[i].first;
    v[i].second = 0;
  }
  REP(i,N){
    cin >> v[N+i].first;
    v[N+i].second = 1;
  }
  sort(v.begin(),v.end());
  int index;
  ll count=0,max=0,min=0;
  pair<int,int> p;

  vector<int> d(v.size());
  REP(i,v.size()){
    if(!v[i].second)count++;
    else count--;
    if(count >= max){
      max = count;
      index = i;
    }
    if(count < min) min = count;
  }
  ll ans;
  if(max>0) ans = (max-min-1)*L + v[index+1].first; 
  else ans = (max-min)*L + v[0].first;
  cout << ans << endl;
  return 0;
}