Given an n x n integer matrix, return the sum of its diagonal elements.

Only include all elements from the primary diagonal and the non-overlapping elements from the secondary diagonal.

Example 1:

Input: matrix = [[3,4,6],
                 [8,9,11],
                 [20,14,10]]
Output: 48
Explanation: Diagonals sum: 3 + 9 + 10 + 6 + 20 = 48
Note that the element matrix[1][1] = 9 is counted only once in the sum.

Example 2:

Input: matrix = [[1,1,1,1],
                 [1,1,1,1],
                 [1,1,1,1],
                 [1,1,1,1]]

Output: 8
Primary diagonal elements: matrix[0][0] = 1, matrix[1][1] = 1, matrix[2][2] = 1, matrix[3][3] = 1
Sum = 1 + 1 + 1 + 1 = 4
Secondary diagonal elements: matrix[0][3] = 1, matrix[1][2] = 1, matrix[2][1] = 1, matrix[3][0] = 1
Sum = 1 + 1 + 1 + 1 = 4
No overlapping elements since it's an even-sized matrix.
Total sum = 4 + 4 = 8

Example 3:

Input: matrix = [[1,1,1,1,1], 
                 [1,1,1,1,1], 
                 [1,1,1,1,1], 
                 [1,1,1,1,1], 
                 [1,1,1,1,1]] 
Output: 9
Primary diagonal elements Sum = 1 + 1 + 1 + 1 + 1 = 5
Secondary diagonal elements Sum = 1 + 1 + 1 + 1 + 1 = 5
The center element matrix[2][2] = 1 is counted twice, so we subtract it once.
Final Calculation: 5 + 5 − 1 = 9

Example 4:

Input: matrix = [[4]]
Output: 4
Explanation: The only element is on both diagonals, so it is counted once.

Constraints

matrix.length == n

matrix[i].length == n for every valid row index i

1 <= n <= 100

1 <= matrix[i][j] <= 100

15. Sum of Diagonal Elements