#include <bits/stdc++.h>

using namespace std;

using ll = long long;

constexpr char newl = '\n';
constexpr ll INF = 1e17;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int n;
    ll K;
    cin >> n >> K;

    vector<ll> a(n), b(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    for (int i = 0; i < n; i++) {
        cin >> b[i];
    }

    vector<ll> dp(n, INF);
    dp[0] = 0;

    ll ans = 0;
    for (int i = 1; i < n; i++) {
        dp[i] = min(dp[i], dp[i - 1] + a[i - 1] + b[i]);
        if (i > 1) dp[i] = min(dp[i], dp[i - 2] + a[i - 2] + b[i] + K);
        //cerr << dp[i] << newl;
        ans = max(ans, dp[i]);
    }
    cout << ans << newl;

    return 0;
}