class Program
{
    static void Main(string[] args)
    {
        string line = System.Console.ReadLine().Trim();
        string[] Data = line.Split(' ');

        int K = int.Parse(Data[0]);
        int N = int.Parse(Data[1]);
        int F = int.Parse(Data[2]);

        int totalbeans = K * N;
        line = System.Console.ReadLine().Trim();
        Data = line.Split(' ');

        for (int i = 0; i < F; i++)
        {
            int age = int.Parse(Data[i]);

            totalbeans = totalbeans - age;

            if (totalbeans < 0)
            {
                System.Console.WriteLine(-1);
                return;
            }
        }


        System.Console.WriteLine(totalbeans);
        return;
    }
}