Matrix Diagonal Sum

class Solution:
    def diagonalSum(self, mat: List[List[int]]) -> int:        
        total = 0
        N = len(mat)
        for ix in range(N):
            total = total + mat[ix][ix]
            total = total + mat[ix][N - 1 - ix]
        return total - (mat[N // 2][N // 2] if N % 2 else 0)