Sum of Both Diagonals

A square grid of numbers is given as a list of rows, each row a list of integers, with as many rows as columns. Write diagonal_sum(grid) that adds up the numbers on its two diagonals: the one from the top-left corner down to the bottom-right corner, and the one from the top-right corner down to the bottom-left corner.

When the grid has an odd size, the two diagonals cross in the centre cell: count that cell once. Return the total as an integer.

Examples

diagonal_sum([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])        # returns 25
diagonal_sum([[1, 2],
              [3, 4]])           # returns 10
diagonal_sum([[7]])              # returns 7

In the first grid the diagonals are 1, 5, 9 and 3, 5, 7. The 5 sits on both, so the total is 1 + 5 + 9 + 3 + 7 = 25.

Constraints

• The grid has 1 to 100 rows and the same number of columns.
• Values are between -1,000 and 1,000.