Vault Grid Diagonal Audit

A vault's floor is tiled with an n×n grid of pressure-sensor readings. For a security audit you must report the total of every sensor lying on either diagonal of the grid: the main diagonal running from the top-left corner to the bottom-right corner, and the anti-diagonal running from the top-right corner to the bottom-left corner. When n is odd, the single sensor at the exact centre of the grid lies on both diagonals — count its reading only once in the total.

Input format

• Line 1: a single integer n.
• Next n lines: each contains n space-separated integers, the i-th line giving row i of the grid (0-indexed).

Output format

A single integer: the sum of every value on the main diagonal plus every value on the anti-diagonal, with the centre cell (when n is odd) counted exactly once.

Constraints

• 1 ≤ n ≤ 100
• -1000 ≤ grid[i][j] ≤ 1000

3
1 2 3
4 5 6
7 8 9

25

1
5

5