static void Main(string[] args)
        {
            int i, j, k = 0, l;
            int n = 4;
            int sum = 0;
            int[,] arr = new int[n,n];
            int[] arr1 = new int[n*n];
            Random r = new Random(24115);
            for (i = 0; i < n; i++)
            {
                for (j = 0; j < n; j++)
                {
                    arr[i, j] = r.Next(-5, 9);
                    arr1[k] = arr[i, j];
                    k++;
                    Console.Write($"{arr[i,j],3}");
                }
                Console.WriteLine();
            }
            j = 0;
            k = 0;
            l = 0;
            while (k < n * n)
            {
                l++;
                j = k + l;
                for (i = k; i < j; i++)
                {
                    sum += arr1[i];
                }

                k += n;
            }
            Console.WriteLine(sum);
        }