C# Count frequncies of elements in array

C# Count frequncies of elements in array

Array of length n having integers 1 to n with  some elements being repeated. 
Count frequencies of all elements from 1 to n in Time Complexity O(n) and Space Complexity O(1)

 class Program
    {
        public static void Main(string[] args)
        {
            var matrix = new int[] { 5, 2, 7, 7, 5, 5, 2 };

            var someUniqueNumberGreaterThan = matrix.Max() + 1;

            for (int i = 0; i < matrix.Length; i++)
            {
                matrix[i]--;
            }

            for (int i = 0; i < matrix.Length; i++)
            {
                var value = matrix[i];

                var originalValue = -1;

                if (value > someUniqueNumberGreaterThan)
                {
                    originalValue = matrix[i] % someUniqueNumberGreaterThan;
                }

                if (originalValue != -1)
                {
                    matrix[originalValue] = matrix[originalValue] + someUniqueNumberGreaterThan;
                }
                else
                {
                    matrix[value] = matrix[value] + someUniqueNumberGreaterThan;
                }
            }

            for (int i = 0; i < matrix.Length; i++)
            {
                Console.WriteLine($"{i + 1} : {matrix[i]/ someUniqueNumberGreaterThan}");
            }
            Console.Read();
        }
    }