#include <iostream>
#include <algorithm>
#include <set>
#define int long long
#define rep(i, n) for(i = 0; i < n; i++)
using namespace std;

int n, l;
int x[100000];
int y[100000];

int simurate(int last) {
	set<int> A;
	set<int> B;
	set<int>::iterator it;
	int i;
	
	rep(i, n) if (i > 0) A.insert(x[i]);
	rep(i, n) if (i != last) B.insert(y[i]);
	
	int now = x[0];
	int ret = 0;
	
	rep(i, 2 * n - 2) {
		if (i % 2 == 0) {
			it = B.lower_bound(now);
			if (it == B.end())
				it = B.lower_bound(0);
			int nxt = (*it);
			if (nxt < now) {
				ret++;
			}
			B.erase(it);
			now = nxt;
		}
		else {
			it = A.lower_bound(now);
			if (it == A.end())
				it = A.lower_bound(0);
			int nxt = (*it);
			if (nxt < now) {
				ret++;
			}
			A.erase(it);
			now = nxt;
		}
	}
	
	if (y[last] < now) ret++;
	return ret;
}

signed main() {
	int i;
	
	cin >> n >> l;
	rep(i, n) cin >> x[i];
	rep(i, n) cin >> y[i];
	
	int okimoti = simurate(n - 1);
	int st = -1, ed = n - 1, mid;	//(st, ed], xxxooo
	while (ed - st >= 2) {
		mid = (st + ed) / 2;
		if (simurate(mid) > okimoti) st = mid;
		else ed = mid;
	}
	
	int ans = simurate(ed) * l + y[ed];
	cout << ans << endl;
	return 0;
}