#nullable enable

using System.Numerics;

void Run()
{
    var n = Int();
    var ti = Int();
    var tz = (n - 1).Repeat(Int);
    var k = Int();
    var xz = new bool[n];
    for (var i = 0; i < k; i++) xz[Int() - 1] = true;
    var ans = n;
    var fail = -1;
    bool F(int m)
    {
        var l = ti;
        for (var i = 1; i < n; i++)
        {
            var t = tz[i - 1];
            l -= t;
            if (l <= 0) return false;
            if (xz[i] && m > 0)
            {
                l += 10;
                m--;
            }
        }
        return true;
    }
    while (ans - fail >= 2)
    {
        var mid = (fail + ans) / 2;
        if (F(mid)) ans = mid;
        else fail = mid;
    }
    if (ans > k) ans = -1;
    Out(ans);
}

#region
AtCoderIO _io_;
var _backend_ = new StandardIOBackend();
_io_ = new(){ Backend = _backend_ };
Run();
_backend_.Flush();

string String() => _io_.Next();
int Int() => int.Parse(String());
void Out(object? x, string? sep = null) => _io_.Out(x, sep);

class AtCoderIO
{
    public required StandardIOBackend Backend { get; init; }

    Memory<string> _input = Array.Empty<string>();
    int _iter = 0;
    public string Next()
    {
        while (_iter >= _input.Length) (_input, _iter) = (Backend.ReadLine().Split(' '), 0);
        return _input.Span[_iter++];
    }

    public void Out(object? x, string? separator = null)
    {
        if (x == null) return;
        separator ??= Environment.NewLine;
        if (x is System.Collections.IEnumerable a and not string)
        {
            var objects = a.Cast<object>();
            if (separator == Environment.NewLine && !objects.Any()) return;
            x = string.Join(separator, objects);
        }
        Backend.WriteLine(x);
    }
}

class StandardIOBackend
{
    readonly StreamReader _sr = new(Console.OpenStandardInput());
    readonly StreamWriter _sw = new(Console.OpenStandardOutput()) { AutoFlush = false };
    public string ReadLine() => _sr.ReadLine()!;
    public void WriteLine(object? value) => _sw.WriteLine(value);
    public void Flush() => _sw.Flush();
}
#endregion

static class Extensions
{
    public static T[] Repeat<T>(this int time, Func<T> F) => Enumerable.Range(0, time).Select(_ => F()).ToArray();
}