Unique Paths II

Follow up for "Unique Paths":
Now consider if some obstacles are added to the grids. How many unique paths would there be?
An obstacle and empty space is marked as 1 and 0 respectively in the grid.
There is one obstacle in the middle of a 3x3 grid as illustrated below.
The total number of unique paths is 2.
m and n will be at most 100.

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