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;
class Program
{
    //static List<string> str = new List<string>();
	static void Main(string[] args)
	{
		// 入力フェーズ
		int N = Int32.Parse(Console.ReadLine());
		string[] A = Console.ReadLine().Split(' '); // 街にいるモンスターの数
		string[] B = Console.ReadLine().Split(' ');// 勇者の倒せるモンスターの数

		List<int> city = new List<int>();
		List<int> brave = new List<int>();
		for(int i=0; i<=N; i++){
			city.Add(Int32.Parse(A[i]));
		}
		for(int i=0; i<N; i++)
		{
			brave.Add(Int32.Parse(B[i]));
		}
		// 勇者が全力で倒して行くと最大になるから・・・
		long monster = 0;
		for(int i=0; i<N; i++)
		{
				// i番目の勇者が倒せる数は・・・
				int cityMonster1 = city[i];
				int braveNum = brave[i];

				if(cityMonster1 >= braveNum)
				{
					// 最初の街で力尽きた
					monster += braveNum;
					continue;
				}
				else
				{
					// 余力あり
					monster += cityMonster1;
					braveNum -= cityMonster1;
				}



				// i+1番目の街のモンスターも倒す
				int cityMonster2 = city[i+1];
				if(cityMonster2 >= braveNum)
				{
					// 2番目の街で力つきた
					monster += braveNum;
					//ほうれんそう
					city[i+1] = cityMonster2 - braveNum;
				}
				else
				{
					// 2番目の街も片付けた
					monster += cityMonster2;
					city[i+1] = 0;
				}
		}
    // 最終的に倒せたモンスターは・・・
		Console.WriteLine(monster + "");


    }

}