#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N, L;
	cin >> N >> L;
	vector<int> A(N);
	vector<int> B(N);
	int res = 0;
	for (int i = 0; i < N; i++) {
		cin >> A[i];
	}
	for (int i = 0; i < N; i++) {
		cin >> B[i];
	}

	vector<pair<int, int> > vp;
	for (int i = 0; i < N; i++) {
		vp.emplace_back(A[i], i);
		vp.emplace_back(B[i], i + N);
	}

	sort(vp.begin(), vp.end());
	int cur = 0;
	N = 2 * N;
	res = -(1ll << 50);
	for (int ii = 0; ii < N; ii++) {
		int i = ii % N;

		//cerr << vp[i].first << " " << vp[i].second << endl;
		if (vp[i].second < N / 2) {
			cur++;
		}
		else {
			cur--;
			if (cur < 0){
				cur = 0;
				res = max(res, vp[i].first);
				res += L;
				
			}
			else {
				int tmp = cur * L + vp[i].first;
				res = max(res, tmp);
			}
		}
	}








	cout << res << endl;
}