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

int main(){
    int N, K;
    cin >> N >> K;
    vector<int64_t> 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<int64_t> dp(N);
    dp[0] = 0;
    for(int i=1; i<N; i++){
        dp[i] = dp[i-1] + A[i-1] + B[i];
        if(i > 2) dp[i] = min(dp[i], dp[i-2] + A[i-2] + B[i] + K);
    }
    int64_t ans = *max_element(dp.begin(), dp.end());
    cout << ans << endl;
    return 0;
}