using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var t = int.Parse(line[1]);
        line = Console.ReadLine().Trim().Split(' ');
        var a = Array.ConvertAll(line, int.Parse);
        Console.ReadLine();
        line = Console.ReadLine().Trim().Split(' ');
        var b = Array.ConvertAll(line, int.Parse);
        getAns(n, t, a, b);
    }

    static void getAns(int n, int t, int[] a, int[] b)
    {
        var c = new bool[n];
        foreach (var x in b) c[x - 1] = true;
        var use = 0;
        var p = 0;
        for (int i = 0; i < n - 1; i++)
        {
            if (c[i]) p++;
            t -= a[i];
            if (t <= 0)
            {
                var need = (-t) / 10 + 1;
                if (p < need) { Console.WriteLine(-1); return; }
                else { p -= need; use += need; t += need * 10; }
            }
        }
        Console.WriteLine(use);
    }
}