#include <iostream>
#include <vector>
#include <algorithm>
#include <cassert>
#include <set>
using namespace std;
typedef long long ll;
typedef pair<ll,int> P;
ll N,L,X[100010] = {},Y[100010] = {};
vector<P> v,u;
int main(){
	cin >> N >> L;
	set<ll> s;
	assert(N<=100000 && L<=1000000000);
	for(int i=1;i<=N;i++){
		cin >> X[i];
		assert(X[i-1]<X[i]);
		assert(1<=X[i] && X[i]<L);
		s.insert(X[i]);
	}
//	assert(s.size()==N);
	for(int i=1;i<=N;i++){
		cin >> Y[i];
		assert(Y[i-1]<Y[i]);
		assert(1<=Y[i] && Y[i]<L);
		s.insert(Y[i]);
	}
	assert((int) s.size()==2*N);
	for(int i=1;i<=N;i++){
		v.push_back(P(X[i],1));
		v.push_back(P(Y[i],-1));
	}
	sort(v.begin(),v.end());
	ll ma = 0,mi = 1e9,p = 0,q = 0;
	ll cnt = 0;
	for(int i=0;i<2*N;i++){
		cnt += v[i].second;
		if(ma<=cnt){
			ma = cnt; p = i;
		}
		if(mi>cnt){
			mi = cnt; q = i;
		}
	}
	cerr << ma << " " << mi << endl;
	if(mi>=0){
		cout << (ma-1)*L + v[p+1].first << endl;
		return 0;
	}
	bool sushi = false;
	for(int i=0;i<=q;i++){
		if(v[i].second==1){
			if(sushi) u.push_back(P(v[i].first+L,1));
			else sushi = true;
		}else{
			if(sushi && i==q) sushi = false;
			else u.push_back(P(v[i].first+L,-1));
		}
	}
	for(int i=q+1;i<2*N;i++) u.push_back(v[i]);
	sort(u.begin(),u.end());
	ma = 0; p = 0; cnt = 0;
	assert(u.size()==2*N-2);
	for(int i=0;i<(int) u.size();i++){
		cnt += u[i].second;
		if(ma<=cnt){
			ma = cnt; p = i;
		}
	}
	cerr << ma << " " << endl;
	cout << (ma-1)*L + u[p+1].first << endl;
	return 0;
}