Given a `m x n` matrix `grid` which is sorted in non-increasing order both row-wise and column-wise, return the number of negative numbers in `grid`.

Example 1:

    Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]]
    Output: 8
    Explanation: There are 8 negatives number in the matrix.

Example 2:

    Input: grid = [[3,2],[1,0]]
    Output: 0

Constraints:

- `m == grid.length`
- `n == grid[i].length`
- `1 <= m, n <= 100`
- `-100 <= grid[i][j] <= 100`

Follow up: Could you find an `O(n + m)` solution?