class Program
    {
        static void Main(string[] args)
        {
            string[] numList = Console.ReadLine().Split(' ');
            string[] ageList = Console.ReadLine().Split(' ');
            
            Console.WriteLine(BeansCount(int.Parse(numList[0]) * int.Parse(numList[1]), ageList));
        }

        private static int BeansCount(int beans, string[] ageList)
        {
            int i = 0;
            while (i < ageList.Length && beans >= 0)
            {
                beans -= int.Parse(ageList[i]);
                i++;
            }
            if (beans < 0 || beans == 0 && i != ageList.Length)
            {
                beans = -1;
            }
            return beans;
        }
    }