import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n + 1];

		for (int i = 0; i <= n; i++) {
			a[i] = sc.nextLong();
		}

		long h = 0;
		long b;
		long d = 0;
		for (int i = 0; i <= n; i++) {
			if (i!=n) {
				b = sc.nextLong();
			} else {
				b = 0;
			}
			if (h <= a[i]) {
				d += h;
				a[i] -= h;
			} else  {
				d += a[i];
				a[i] = 0;
			}
			if (b <= a[i]) {
				d += b;
				h = 0;
			} else {
				d += a[i];
				h = b - a[i];
			}
		}
		System.out.println(d);
	}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz