Problem:
Score : 300 points

Problem Statement

There are N+1 towns. The i-th town is being attacked by A_i monsters.

We have N heroes. The i-th hero can defeat monsters attacking the i-th or (i+1)-th town, for a total of at most B_i monsters.

What is the maximum total number of monsters the heroes can cooperate to defeat?

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^9

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_{N+1}
B_1 B_2 ... B_N

Output

Print the maximum total number of monsters the heroes can defeat.

Sample Input 1

2
3 5 2
4 5

Sample Output 1

9

If the heroes choose the monsters to defeat as follows, they can defeat nine monsters in total, which is the maximum result.

The first hero defeats two monsters attacking the first town and two monsters attacking the second town.

The second hero defeats three monsters attacking the second town and two monsters attacking the third town.

Sample Input 2

3
5 6 3 8
5 100 8

Sample Output 2

22

Sample Input 3

2
100 1 1
1 100

Sample Output 3

3

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
 
public class Solution
{
    public static void Main()
    {
      long N = long.Parse(Console.ReadLine());
      string str1 = Console.ReadLine();
      string str2 = Console.ReadLine();
      string[] strA = str1.Split(' ');
      string[] strB = str2.Split(' ');
      long[] A = new long[N+1];
      long[] B = new long[N];
      for (int i = 0; i < N; i++)
      {
        A[i] = long.Parse(strA[i]);
        B[i] = long.Parse(strB[i]);
      }
      A[N] = long.Parse(strA[N]);
      
      // N+1番目の町はN番目の勇者しか倒せないので
      // N+1からやっていけばよさそう
      long total = 0;
      long amari = 0;
      for (long i = N; i > 0L; i--)
      {
        // 一つ前の処理でN+1番目の勇者が残した倒せる数を処理する
        // モンスターの数が同じか残る場合、
        //  totalに勇者が残した倒せる数を足す、モンスターの数から倒した数をひく
        // モンスターの数より倒せる場合、totalにモンスターの数を足す、モンスターの数を0にする
        if (A[i] - amari >= 0)
        {
          total += amari;
          A[i] -= amari;
        }
        else
        {
          total += A[i];
          A[i] = 0;
        }
        
        // N番目の勇者が倒せるモンスターの数を処理する
        // モンスターの数が同じか残る場合、
        //  totalに勇者が残した倒せる数を足す、勇者あまりを0にする
        // モンスターの数より倒せる場合、
        //  totalにモンスターの数を足す、勇者あまりを勇者からモンスターを引く
        if (A[i] - B[i-1] >= 0)
        {
          total += B[i-1];
          amari = 0;
        }
        else
        {
          total += A[i];
          amari = B[i-1] - A[i];
        }
      }
      // 1番目の町の処理
      if (A[0] - amari >= 0)
      {
        total += amari;
        A[0] -= amari;
      }
      else
      {
        total += A[0];
        A[0] = 0;
      }      
      Console.WriteLine($"{total}");
//      Console.WriteLine($"{p[0]} {p[1]} {p[2]} {p[3]} {p[4]}");
//      Console.WriteLine($"{dst[0]} {dst[1]} {dst[2]} {dst[3]} {dst[4]}");
//      Console.WriteLine($"{cnt}");
    }
}