static int IntegersToCompleteArray(int[] testArray)
        {   // before sorting array [ 9, 0, 3]
            Array.Sort(testArray);
            // after sorting array [ 0, 3, 9]
            int counter = 0;

            for (int i = 0; i < testArray.Length - 1; i++)
            {
                counter += testArray[i + 1] - testArray[i] - 1;
                //first iteration 0 += 3 - 0 -1 = 2
                //second iterations 2 += 9 - 3 - 1 = 7
                //third iteration 7 += ? - 9 - 1 iteration doesn't go?
                Console.WriteLine(counter);
            }

            return counter;
        }