using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    internal class Program
    {
        static Random rn = new Random();

        static void Main(string[] args)
        {
            string[] be = Console.ReadLine().Split(' ');
            int N = int.Parse(be[0]), M = int.Parse(be[1]), jo = 0;

            int[] napok = new int[N+1];
            int[] munkak = new int[N];

            be = Console.ReadLine().Split(' ');
            for (int i = 0, j; i < N; i++)
            {
                napok[i] = int.Parse(be[i]);
                munkak[i] = 0;
            }

            for (int i = 0, j; i < M; i++)
            {
                j = int.Parse(Console.ReadLine());
                munkak[j - 1]++;
            }

            for (int i = 0; i < N; i++)
            {
                jo += Math.Min(napok[i], munkak[i]);
                napok[i + 1] += Math.Max(0, napok[i] - munkak[i]);
            }

            Console.WriteLine(jo);
            //Console.ReadKey();
        }
    }
}