#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int n,l;
  cin>>n>>l;
  set<Int> sx,sy;
  for(Int i=0;i<n;i++){
    Int x;
    cin>>x;
    sx.emplace(x);
  }
  for(Int i=0;i<n;i++){
    Int y;
    cin>>y;
    sy.emplace(y);
  }
  Int time=0;
  while(!sx.empty()){
    {
      Int t=time%l;
      auto it=sx.upper_bound(t);
      if(it==sx.end()){           
        time+=l-t;
        time+=*sx.begin();
        sx.erase(sx.begin());
      }else{      
        time+=*it-t;
        sx.erase(it);
      }
    }
    if(sx.empty()){
      Int t=time%l;
      Int u=*sy.begin();
      if(t<u){
        time+=u-t;
      }else{
        time+=l-t;
        time+=u;
      }
      break;
    }
    {
      Int t=time%l;
      Int u=0;      
      {
        auto it=sy.upper_bound(t);
        if(it==sy.end()) u=*sy.begin();
        else u=*it;
      }
      {
        auto it=sx.upper_bound(u);
        if(it==sx.end()) u=*sx.begin();
        else u=*it;        
      }
      auto it=sy.upper_bound(u);
      if(it==sy.begin()){
        if(t<*sy.rbegin()){
          time+=*sy.rbegin()-t;
        }else{
          time+=l-t;
          time+=*sy.rbegin();
        }
        sy.erase(--sy.end());        
      }else{        
        --it;
        if(t<*it){
          time+=*it-t;
        }else{
          time+=l-t;
          time+=*it;
        }
        sy.erase(it);
      }
    }
  }  
  cout<<time<<endl;
  return 0;
}