test 3 zad 3 1 grupa

class Program
    {
        static void Main(string[] args)
        {
            enterA:
            Console.Write("Enter a: ");
            int a = int.Parse(Console.ReadLine());
            if (a <= 0)
            {
                goto enterA;
            }

            enterB:
            Console.Write("Enter b: ");
            int b = int.Parse(Console.ReadLine());
            if (b <= 0)
            {
                goto enterB;
            }

            Console.WriteLine(F(a + b) * F(Math.Abs(a - b)));
        }

        private static double F(int n)
        {
            double sum = 1;
            for (int i = 2; i <= n; i++)
            {
                sum += (1 / Math.Pow(i, 4));
            }

            return sum;
        }
    }