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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using static System.Console;
using static System.Math;

namespace ABC135
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = ReadInt();
            var a = ReadInts();// 0 - n+1 mosnter
            var b = ReadInts();// 0 - n   braver skill
            int cnt = 0;
            int carry = 0;

            for (int i = 0; i < n; i++)
            {
                var tmp = b[i] + carry; // i-1番目の勇者の余力を加算

                if (tmp > a[i])
                {
                    cnt += a[i];
                    carry = tmp - a[i];
                    if (carry > b[i]) // i-1番目の勇者の余力を削ぐ
                    {
                        carry = b[i];
                    }
                }
                else
                {
                    cnt += tmp;
                    carry = 0;
                }
                //WriteLine("{0} {1}", cnt, carry);
            }

            if (carry>0)
            {
                cnt += (carry > a[n]) ? a[n] : carry;
            }
            WriteLine("{0}", cnt);
            Read();
        }
        private static string Read() { return ReadLine(); }
        private static char[] ReadChars() { return Array.ConvertAll(Read().Split(), a => a[0]); }
        private static int ReadInt() { return int.Parse(Read()); }
        private static long ReadLong() { return long.Parse(Read()); }
        private static double ReadDouble() { return double.Parse(Read()); }
        private static int[] ReadInts() { return Array.ConvertAll(Read().Split(), int.Parse); }
        private static long[] ReadLongs() { return Array.ConvertAll(Read().Split(), long.Parse); }
        private static double[] ReadDoubles() { return Array.ConvertAll(Read().Split(), double.Parse); }

    }
}