#include <bits/stdc++.h>
using namespace std;

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

    int N;
    long long K;
    cin >> N >> K;

    vector<long long> A(N), B(N);
    for (long long& x : A) cin >> x;
    for (long long& x : B) cin >> x;

    // T_1 = 0
    long long answer = 0;

    if (N == 1) {
        cout << 0 << '\n';
        return 0;
    }

    // 0-indexed:
    // prev2 = T_1, prev1 = T_2
    long long prev2 = 0;
    long long prev1 = A[0] + B[1];

    answer = max(answer, prev1);

    for (int i = 2; i < N; ++i) {
        // 社員 i+1 が社員 i の声を聞く時刻
        long long by_previous =
            prev1 + A[i - 1] + B[i];

        // 社員 i-1 の声を聞いてから K 秒後
        long long by_timeout =
            prev2 + A[i - 2] + B[i] + K;

        long long current = min(by_previous, by_timeout);

        // 「全員」が発声するまでなので全 T_i の最大を取る
        answer = max(answer, current);

        prev2 = prev1;
        prev1 = current;
    }

    cout << answer << '\n';
    return 0;
}