#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}

template<typename T> using pqueue_inv = std::priority_queue<T, std::vector<T>, std::greater<T> >;

int main() {
	int n = ri();
	int additional = ri();
	int a[n];
	for (auto &i : a) i = ri(), assert(i);
	int b[n];
	for (auto &i : b) i = ri(), assert(i);
	
	std::vector<int64_t> dist(n, 1000000000000000000);
	pqueue_inv<std::pair<int64_t, int> > que;
	que.push({dist[0] = 0, 0});
	while (que.size()) {
		auto i = que.top();
		que.pop();
		if (i.first != dist[i.second]) continue;
		int cur = i.second;
		if (cur != n - 1 && dist[cur + 1] > i.first + a[cur] + b[cur + 1])
			que.push({      dist[cur + 1] = i.first + a[cur] + b[cur + 1], cur + 1});
		if (cur < n - 2 && dist[cur + 2] > i.first + a[cur] + b[cur + 2] + additional)
			que.push({     dist[cur + 2] = i.first + a[cur] + b[cur + 2] + additional, cur + 2});
	}
	printf("%" PRId64 "\n", *std::max_element(dist.begin(), dist.end()));
	
	return 0;
}