#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
using namespace std;
#define MOD (long long int)(1e9+7)
#define ll long long int
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define reps(i,n) for(int i=1; i<=(int)(n); i++)
#define REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)

//struct position{int x;};

int main(void){
	int n,x,y;
	ll l;
	cin>>n>>l;
	//vector<position> P;
	vector<int> sushi,tea;
	rep(i,n){
		cin>>x;
		sushi.push_back(x);
	}
	rep(i,n){
		cin>>y;
		tea.push_back(y);
	}
	sushi.push_back(1000000009);
	tea.push_back(1000000009);
	vector<int> atode;
	int sc=0,tc=0;
	int count=0,ans=0,ansp;
	int si=0,ti=0;
	while(si<n||ti<n){
		if(sushi[si] < tea[ti]){
			sc++;
			si++;
		}else{
			if(sc > 0){
				if(ans <= sc){
					ans = sc;

					ansp = tea[ti];

				}
				sc--;
			}else{
				atode.push_back(ti%n);
			}
			ti++;
		}

		//cout<<sc<<" "<<tc<<" "<<atode.size()<<" "<<ans<<endl;
	}


	
	if(atode.size() == 0){
		cout<<l*(ans-1)+ansp<<endl;
		return 0;
	}
	ti = (atode[atode.size()-1])%n;
	si = 0;
	while(sushi[si]<tea[ti]){
		si = (si+1)%n;
	}
	ti=(ti+1)%n;
	sc = 0;
	tc = 0;
	vector<ll> sushi2,tea2;
	//cout<<"hogea"<<si<<" "<<ti<<endl;
	rep(i,n){
		if(i<si%n){
			sushi2.push_back(l + sushi[si]);
		}else{
			sushi2.push_back(sushi[si]);
		}
		
	}
	rep(i,n){
		if(i<ti%n){
			tea2.push_back(l + tea[ti]);
		}else{
			tea2.push_back(tea[ti]);
		}
		
	}
	rep(i,2*n){
		if(sushi2[si] < tea2[ti]){
			sc++;
			si = (si+1)%n;
		}else{
			if(sc > 0){
				if(ans <= sc){
					ans = sc;
					if(ansp > tea2[ti]){
						ansp = tea2[ti];
					}
					
				}
				sc--;
			}else{
				
			}
			ti=(ti+1)%n;
		}
		//cout<<sc<<" "<<tc<<" "<<atode.size()<<" "<<ans<<" "<<si<<" "<<ti<<endl;
	}
	cout<<l*(ans-1)+ansp<<endl;
	
	

	return 0;
}