class Program
    {
        static void Main(string[] args)
        {
            string[] numList = Console.ReadLine().Split(' ');
            double beans = int.Parse(numList[0]) * int.Parse(numList[1]);
            int count = int.Parse(numList[2]);
            numList = null;
            string[] ageList = Console.ReadLine().Split(' ');
            int i = 0;

            while (i < count && beans >= 0)
            {
                beans -= int.Parse(ageList[i]);
                i++;
            }

            if (beans < 0 || beans == 0 && i != count)
            {
                beans = -1;
            }
            Console.WriteLine(beans);
        }
    }