class Program
    {
        static void Main(string[] args)
        {
            string num = Console.ReadLine();
            string[] numList = num.Split(' ');
            int beans = int.Parse(numList[0]) * int.Parse(numList[1]);
            string age = Console.ReadLine();
            string[] ageList = age.Split(' ');
            
            for(int i = 0; i < int.Parse(numList[2]); i++)
            {
                beans -= int.Parse(ageList[i]);
                if(beans < 0)
                {
                    beans = -1;
                    break;
                }
            }
            Console.WriteLine(beans);
        }
    }