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;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		
		long cnt1 = 0;
		long carry = 0;
		long[] A1 = (long[]) A.Clone();
		for(int i=0;i<=N;i++){
			var t1 = Math.Min(A1[i], carry);
			cnt1 += t1;
			A1[i] -= t1;
			if(i < N){
				var t2 = Math.Min(A1[i], B[i]);
				cnt1 += t2;
				carry = B[i] - t2;
			}
		}
		
		long cnt2 = 0;
		carry = 0;
		long[] A2 = (long[]) A.Clone();
		for(int i=N;i>=0;i--){
			var t1 = Math.Min(A2[i], carry);
			cnt2 += t1;
			A2[i] -= t1;
			if(i - 1 >= 0){
				var t2 = Math.Min(A2[i], B[i - 1]);
				cnt2 += t2;
				carry = B[i - 1] - t2;
			}
		}
		
		
		Console.WriteLine(Math.Max(cnt2, cnt1));
	}
	int N;
	long[] A;
	long[] B;
	public Sol(){
		N = ri();
		A = rla();
		B = rla();
	}

	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);}
	static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));}
	static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));}
	static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));}
}