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;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Collections;

static class Extensions
{
    public static int ToInt(this string s) => int.Parse(s);
    public static long ToLong(this string s) => long.Parse(s);
    public static char ToChar(this string s) => char.Parse(s);
    public static double ToDouble(this string s) => double.Parse(s);
    public static int[] ToIntArray(this string s) => s.Replace(',', ' ').Split(' ').Select(x => x.ToInt()).ToArray();
    public static long[] ToLongArray(this string s) => s.Replace(',', ' ').Split(' ').Select(x => x.ToLong()).ToArray();
    public static double[] ToDoubleArray(this string s) => s.Replace(',', ' ').Split(' ').Select(x => x.ToDouble()).ToArray();
    public static Queue<T> ToQueue<T>(this IEnumerable<T> seq) => new Queue<T>(seq);
    public static Stack<T> ToStack<T>(this IEnumerable<T> seq) => new Stack<T>(seq);
    public static HashSet<T> ToHashSet<T>(this IEnumerable<T> seq) => new HashSet<T>(seq);
}
static class MyMath
{
    static public long GCD(long a, long b) => (a < b) ? GCD(b, a) : ((b == 0) ? a : GCD(b, a % b));
    static public long LCM(long a, long b) => (a > b) ? (a / GCD(a, b) * b) : (b / GCD(a, b) * a);
    static private readonly long _m = 1000000000 + 7;
    static public long Mod(long val) => (val % _m + _m) % _m;
    static private long extGCD(long a, long b, ref long x, ref long y) => (b == 0) ? (a + (x = 1) * 0 + (y = 0) * 0) : (extGCD(b, a % b, ref y, ref x) + (y -= a / b * x) * 0);
    static public long ModInverse(long a, long dummy1 = 0, long dummy2 = 0) => extGCD(a, _m, ref dummy1, ref dummy2) * 0 + Mod(dummy1);    
}

class Program
{
    static string[] ReadLines(int n) => Enumerable.Range(0, n).Select(s => Console.ReadLine()).ToArray();

    static void Main()
    {
        int N = Console.ReadLine().ToInt();

        var cityList = Console.ReadLine().ToIntArray();
        var yuushaList = Console.ReadLine().ToIntArray();

        //1番目の街のモンスターを倒せるのは1番目の勇者だけだから、1番目の勇者は1番目の街を優先的に倒す。

        long ans = 0;
        for (int i = 0; i < N; i++)
        {
            var capacity = yuushaList[i];

            if (cityList[i] > 0)
            {
                var val = Math.Min(capacity, cityList[i]);                
                ans += val;
                cityList[i] -= val;
                capacity -= val;
            }
            if (cityList[i + 1] > 0)
            {
                var val = Math.Min(capacity, cityList[i + 1]);
                ans += val;
                cityList[i + 1] -= val;
                capacity -= val;
            }
        }

        Console.WriteLine(ans);
    }
    
}