#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cout << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cout<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 300010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = 1 << 30;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
///////////////////////////////////////////////////////////////////////////////////////////////////
int N; ll L;
ll X[MAX_N], Y[MAX_N];
vl vx;
int D[MAX_N];


void solve() {
	cin >> N >> L;
	rep(i, 0, N) {
		cin >> X[i];
		vx.pb(X[i]);
	}
	rep(i, 0, N) {
		cin >> Y[i];
		vx.pb(Y[i]);
	}
	sort(all(vx));

	rep(i, 0, N) {
		int at = lower_bound(all(vx), X[i]) - vx.begin();
		D[at + 1]++;
	}
	rep(i, 0, N) {
		int at = lower_bound(all(vx), Y[i]) - vx.begin();
		D[at + 1]--;
	}
	vx.pb(L);

	int maxv = 0, minv = 0;
	int mat = 0;

	rep(i, 1, 2 * N + 1) {
		D[i] += D[i - 1];
		if(D[i] >= maxv) mat = i;
		MAX(maxv, D[i]);
		MIN(minv, D[i]);
	}
	cout << L * (maxv - minv - 1) + vx[mat] << "\n";
	
}


int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
	srand((unsigned int)time(NULL));
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
    freopen("in.txt", "rt", stdin);
#endif	
	solve();
#ifdef LOCAL
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
#endif
	return 0;
}