Imaging the (m x n) matrix to be a declining triangle,

then the point (m, n) will be at row (m+n-2) and col (n-1),

so its value can be easily calculated by the definition of Pascal's Triangle

```
from math import *
class Solution(object):
def uniquePaths(self, m, n):
"""
:type m: int
:type n: int
:rtype: int
"""
return factorial(m+n-2)/factorial(n-1)/factorial(m-1)
```