✅ This shouldn't work for 2 and 3 but it does

static void Plus(int number)
        {
            Func<int, int> addone = x => (number + 1) / 6;
            Func<int, int> minusone = y => (number - 1) / 6;

            if (addone(number) % 6 != 0 || minusone(number) % 6 != 0)
            {
                Console.WriteLine("{0} is not prime", number);
            }
            else
            {
                Console.WriteLine("{0} is prime", number);
            }
        }