Given a square matrix mat of size n × n, return the sum of the elements on both diagonals — the primary diagonal (top-left to bottom-right) and the secondary diagonal (top-right to bottom-left). If the matrix has an odd number of rows and columns, the very center cell sits on both diagonals at once — count it only once. Checking every cell is correct but wasteful: the diagonal positions are known in advance by a simple formula, so a single pass that visits only those 2n (or 2n-1) cells finds the answer in O(n) instead of O(n²).