static int Sum3X3(int[,]arr, int row, int col)
        {
            int result = 0;

            for (int r = row; r < row + 3; r++)
            {
                for (int c = col; c < col + 3; c++)
                {
                    result += arr[r, c];
                }
            }

            return result;
        }// int Sum3X3(arr, row, col)