The following code works, while I change to the second fomular it does not work

```
def numTrees(self, n):
res = 1
for i in xrange(1, n+1):
res = res*2*(2*i-1)/(i+1) # Works
return res
def numTrees(self, n):
res = 1
for i in xrange(1, n+1):
res *= 2*(2*i-1)/(i+1) # Does not work
return res
```