import java.util.*;
import java.util.stream.*;

public class Main {
  public static void main(String[] args) throws Exception {
    Scanner in = new Scanner(System.in);
    int N = in.nextInt();
    long[] A = new long[N + 1];
    long[] B = new long[N];
    for (int i = 0; i < N + 1; i++) {
      A[i] = in.nextLong();
    }
    for (int i = 0; i < N; i++) {
      B[i] = in.nextLong();
    }
    long ans = 0;
    for (int i = 0; i < N; i++) {
      ans += Math.min(A[i], B[i]);
      B[i] -= Math.min(A[i], B[i]);
      ans += Math.min(A[i + 1], B[i]);
      A[i + 1] -= Math.min(A[i + 1], B[i]);
    }
    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