#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;

struct Node {
  int id;
  ll t;

  Node(int id = -1, ll t = -1) {
    this->id = id;
    this->t = t;
  }

  bool operator>(const Node &n) const {
    return t > n.t;
  }
};

int main() {
  ll N, K;
  cin >> N >> K;

  vector<ll> A(N);
  vector<ll> B(N);
  for (int i = 0; i < N; ++i) {
    cin >> A[i];
  }
  for (int i = 0; i < N; ++i) {
    cin >> B[i];
  }

  priority_queue <Node, vector<Node>, greater<Node>> pque;
  vector<bool> visited(N, false);
  ll ans = 0;
  pque.push(Node(0, 0));

  while (not pque.empty()) {
    Node node = pque.top();
    pque.pop();

    if (visited[node.id]) continue;
    visited[node.id] = true;
    if(node.id >= N - 2) {
      ans = max(ans, node.t);
    }

    if (node.id + 1 < N) {
      pque.push(Node(node.id + 1, node.t + A[node.id] + B[node.id + 1]));
    }
    if (node.id + 2 < N) {
      pque.push(Node(node.id + 2, node.t + A[node.id] + B[node.id + 2] + K));
    }
  }

  cout << ans << endl;

  return 0;
}