It's actually (m + n - 2)! / ((m - 1)! * (n - 1)!)

```
class Solution(object):
def uniquePaths(self, m, n):
"""
:type m: int
:type n: int
:rtype: int
"""
if m > n:
m, n = n, m
i = n
j = 1
res = 1
for k in range(m - 1):
res *= i
#res = res * 1.0 / j
i += 1
#j += 1
res2 = 1
for k in range(m - 1):
res2 *= j
j += 1
return int(res / res2)
```