Unique Paths in a Grid

Given a grid of size m * n, lets assume you are starting at (1,1) and your goal is to reach (m,n). At any instance, if you are on (x,y), you can either go to (x, y + 1) or (x + 1, y).

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.

Example :
There is one obstacle in the middle of a 3x3 grid as illustrated below.


The total number of unique paths is 2.

Note: m and n will be at most 100.

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