public int CountElements(int[] arr)
        {
            var dict = new Dictionary<int, int>();
            var sum = 0;
            foreach (var num in arr)
            {
                if (dict.TryGetValue(num, out var count))
                {
                    dict[num] = count + 1;
                }
                else
                {
                    dict.Add(num, 1);
                }
            }

            foreach (var (key, value) in dict)
            {
                if (dict.TryGetValue(key + 1, out var _value))
                {
                    sum += value;
                }
            }

            return sum;
        }