import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] a = new int[n+1];
        int[] b = new int[n];
        long ans = 0;
        for (int i=0;i<=n;i++)a[i]=sc.nextInt();
        for (int i=0;i<n;i++)b[i]=sc.nextInt();
        for (int i=0;i<n;i++){
            int def1 = a[i];
            int def2 = a[i+1];
            a[i] = Math.max(0,a[i]-b[i]);
            a[i+1] = Math.max(0,Math.min(a[i+1],a[i+1]+def1-b[i]));
            ans+=def1-a[i]+def2-a[i+1];
        }
        System.out.println(ans);

    }
}

/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