import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		int n = Integer.parseInt(sa[0]);
		int k = Integer.parseInt(sa[1]);

		sa = br.readLine().split(" ");
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = Integer.parseInt(sa[i]);
		}

		sa = br.readLine().split(" ");
		int[] b = new int[n];
		for (int i = 0; i < n; i++) {
			b[i] = Integer.parseInt(sa[i]);
		}
		br.close();

		long[] dp = new long[n];
		for (int i = 1; i < n; i++) {
			dp[i] = dp[i - 1] + a[i - 1] + b[i];
			if (i > 1) {
				long val = dp[i - 2] + a[i - 2] + b[i] + k;
				dp[i] = Math.min(dp[i], val);
			}
		}

		long ans = 0;
		for (int i = 0; i < dp.length; i++) {
			ans = Math.max(ans, dp[i]);
		}
		System.out.println(ans);
	}
}