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;

namespace debug
{
    public static partial class Program
    {
        public static void Main()
        {
            var n = Console.ReadLine().ToLong();
            var ais = Console.ReadLine().ToLongArray();
            var bis = Console.ReadLine().ToLongArray();

            long cnt = 0;
            for (int i = 0; i < n; i++)
            {
                if (bis[i] > ais[i])
                {
                    cnt += ais[i];
                    bis[i] -= ais[i];
                    ais[i] = 0;

                    if (bis[i] > 0)
                    {
                        if (ais[i + 1] < bis[i])
                        {
                            cnt += ais[i + 1];
                            ais[i] = 0;
                        }
                        else
                        {
                            cnt += bis[i];
                            ais[i + 1] -= bis[i];
                        }
                    }
                }
                else
                {
                    cnt += bis[i];
                    ais[i] = 0;
                }
            }

            Console.WriteLine(cnt);
        }
    }

    public static partial class Program
    {
        //テンプレここから
        private static long GCD(long aaa, long bbb) => bbb == 0 ? aaa : GCD(bbb, aaa % bbb);
        private static long LCM(long aaa, long bbb) => aaa * bbb / GCD(aaa, bbb);
        private static long ToLong(this double ddd) => (long) ddd;
        private static long ToLong(this string str) => long.Parse(str);
        private static long[] ToLong(this IEnumerable<string> strs) => strs.Select(x => x.ToLong()).ToArray();
        private static long[] ToLongArray(this string str, char opr = ' ') => str.Split(opr).ToLong();
        private static long Diff(long aaa, long bbb) => Math.Abs(Math.Max(aaa, bbb) - Math.Min(aaa, bbb));
        private static long Sign(this long iii) => Math.Sign(iii);
        private static bool IsEven(this long iii) => iii % 2 == 0;
        private static readonly int[] Dx = {0, -1, 0, 1};
        private static readonly int[] Dy = {-1, 0, 1, 0};
        public const int MOD = 1000000007;
        public const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        public const string alphabet = "abcdefghijklmnopqrstuvwxyz";
        public static long ToLong(this char ccc) => ccc.ToString().ToLong();
        public static string Join(this IEnumerable<string> array, string sep = "") => string.Join(sep, array);
        public static string Join(this IEnumerable<char> array, string sep = "") => string.Join(sep, array);

        public static string JoinToStr(this IEnumerable<long> array, string sep = "")
            => array.Select(xxx => xxx.ToString()).Join(sep);

        public static string JoinToStr(this IEnumerable<int> array, string sep = "")
            => array.Select(xxx => xxx.ToString()).Join(sep);

        //テンプレここまで
    }
}