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.Linq;
using System.Collections.Generic;

namespace ConsoleApp2
{
    public class Program
    {

        static long D = (long)Math.Pow(10, 9) + 7;

        public static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] monsterNumbers = Console.ReadLine().Split(' ');
            string[] limits = Console.ReadLine().Split(' ');

            int killNumber = 0;
            int nextMonsterNumber = int.Parse(monsterNumbers[0]);

            for (int i = 0; i < N; i++)
            {
                //Console.WriteLine("勇者" + i + "が登場");
                int mNumber = nextMonsterNumber;
                nextMonsterNumber = int.Parse(monsterNumbers[i + 1]);
                int limit = int.Parse(limits[i]);
                if (mNumber >= limit) killNumber += limit;
                else
                {
                    int surplus = limit - mNumber;
                    killNumber += mNumber;
                    if (nextMonsterNumber >= surplus)
                    {
                        nextMonsterNumber -= surplus;
                        killNumber += surplus;
                    }
                    else
                    {
                        killNumber += nextMonsterNumber;
                        nextMonsterNumber = 0;
                    }
                }
            }

            Console.WriteLine(killNumber);
            Console.ReadLine();
        }

        static long Com(int n, int m)
        {
            if (m == 0 || m == n) return 1;

            if (n < m) return 0;

            int[] a = new int[m];

            for (int i = 0; i < m; i++)
            {
                a[i] = n - i;
            }
            for (int i = 1; i <= m; i++)
            {
                for (int j = 0; j < m; j++)
                {
                    if (a[j] % i == 0)
                    {
                        a[j] /= i;
                        break;
                    }
                }
            }

            long ans = 1;
            for (int i = 0; i < m; i++)
            {
                ans *= a[i];
                ans %= D;
            }
            return ans;
        }


        // 最小公倍数
        public static int Lcm(int a, int b)
        {
            return a * b / Gcd(a, b);
        }

        // ユークリッドの互除法 
        public static int Gcd(int a, int b)
        {
            if (a < b)
                // 引数を入替えて自分を呼び出す
                return Gcd(b, a);
            while (b != 0)
            {
                var remainder = a % b;
                a = b;
                b = remainder;
            }
            return a;
        }


        private static bool S(int[] data,int K ,int l ,int x)
        {
            int sum = 0;
            int max = Math.Min(x+1, data.Length);
            for (int i = l; i < max; i++)
            {
                sum += data[i];
            }
            Console.WriteLine(sum);
            return sum>=K;
        }


        public static IEnumerable<T[]> Enumerate<T>(IEnumerable<T> items)
        {
            if (items.Count() == 1)
            {
                yield return new T[] { items.First() };
                yield break;
            }
            foreach (var item in items)
            {
                var leftside = new T[] { item };
                var unused = items.Except(leftside);
                foreach (var rightside in Enumerate(unused))
                {
                    yield return leftside.Concat(rightside).ToArray();
                }
            }
        }




        private static string ReplaceOneChar(string S, int index, string newS)
        {
            string s1 = S.Substring(0, index - 1);
            string s2 = S.Substring(index);
            return s1 + newS + s2;
        }

        private static bool PartialSum(int target, int[] Ns)
        {
            //if (target == 0) return true;
            bool[][] ps = new bool[Ns.Length + 1][];

            //i=0では、どんなWに対してもfalse
            ps[0] = new bool[target+1];
            for (int i = 0; i < ps[0].Length; i++) ps[0][i] = false;

            ps[0][0] = true;


            //考慮に入れるitemを、前から順に追加していく
            for (int i = 0; i < Ns.Length; i++)
            {
                //初期化
                ps[i + 1] = new bool[target+1];

                for (int t = 0; t <= target; t++)
                {
                    if (t >= Ns[i]) ps[i + 1][t] = ps[i][t - Ns[i]] || ps[i][t];
                    else ps[i + 1][t]=ps[i][t];
                }
            }

            return ps[Ns.Length][target];
        }

        /// <summary>
        /// 引数は、W(重さ制限)、items[重さ][価値]
        /// </summary>
        /// <param name="W"></param>
        /// <param name="items"></param>
        /// <returns></returns>
        private static int DP(int W,int[][] items)
        {
            //データ
            int[][] dp = new int[items.Length+1][];

            //i=0では、どんなWに対しても0
            dp[0] = new int[W + 1];

            //考慮に入れるitemを、前から順に追加していく
            for (int i = 0; i < items.Length; i++)
            {
                //初期化
                dp[i+1] = new int[W + 1];

                for (int w = 0; w <= W; w++)
                {
                    if (w >= items[i][0]) dp[i + 1][w] = Math.Max(dp[i][w - items[i][0]]+items[i][1], dp[i][w]);
                    else dp[i + 1][w] = dp[i][w];
                }
            }
            return dp[items.Length][W];
        }

        private static int[] Read(string input)
        {
            string[] strings = input.Split(' ');
            int num = strings.Length;
            int[] ints = new int[num];
            for (int i = 0; i < num; i++)
            {
                ints[i] = int.Parse(strings[i]);
            }
            
            return ints;
        }


    }


}