class Program
{
    static void Main(string[] args)
    {
        string[] input = Console.ReadLine()!.Split(' ');
        string[] inputAge = Console.ReadLine()!.Split(' ');

        int beanCount = int.Parse(input[0]);
        int bagCount = int.Parse(input[1]);
        int ageSum = 0;

        for (int i = 0; i < inputAge.Length; i++)
        {
            if (inputAge[i] != "")
            {
                int age = int.Parse(inputAge[i]);
                ageSum += age;
            }
        }

        int beanSum = beanCount * bagCount;
        int ans = beanSum - ageSum;

        if (ans >= 0)
        {
            Console.WriteLine(ans);
        }
        else
        {
            Console.WriteLine("-1");
        }
    }
}