using System;
using System.IO;
using System.Linq;

namespace Contest
{
    class Program
    {
        private Scanner sc;
        public void Solve()
        {
            sc = new Scanner();
            int N = sc.NextInt();
            int[] A = sc.IntArray();
            int[] B = sc.IntArray();

            long ans = 0;

            for (int i = N - 1; i >= 0; i--)
            {
                if (A[i + 1] < B[i])
                {
                    B[i] -= A[i + 1];
                    ans += A[i + 1];
                    A[i + 1] = 0;
                    if (A[i] < B[i])
                    {
                        ans += A[i];
                        A[i] = 0;
                        B[i] -= A[i];
                    }
                    else
                    {
                        ans += B[i];
                        A[i] -= B[i];
                        B[i] = 0;
                    }
                }
                else
                {
                    A[i + 1] -= B[i];
                    ans += B[i];
                    B[i] = 0;
                }
            }

            ans += Math.Min(A[0], B[0]);
            Console.WriteLine(ans);
        }

        static void Main() => new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _stream = new StreamReader(Console.OpenStandardInput());
        _pos = 0;
        _line = new string[0];
        _separator = ' ';
    }
    private readonly char _separator;
    private readonly StreamReader _stream;
    private int _pos;
    private string[] _line;
    #region get a element
    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = _stream.ReadLine().Split(_separator);
            _pos = 0;
        }
        return _line[_pos++];
    }
    public int NextInt()
    {
        return int.Parse(Next());
    }
    public long NextLong()
    {
        return long.Parse(Next());
    }
    public double NextDouble()
    {
        return double.Parse(Next());
    }
    #endregion
    #region convert array
    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }
    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }
    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }
    #endregion
    #region get array
    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = _stream.ReadLine().Split(_separator);
        _pos = _line.Length;
        return _line;
    }
    public int[] IntArray()
    {
        return ToIntArray(Array());
    }
    public long[] LongArray()
    {
        return ToLongArray(Array());
    }
    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }
    #endregion
}

Measured memory_bytes: 33628