friends pairing problem

f(n) = ways n people can remain single 
       or pair up.

For n-th person there are two choices:
1) n-th person remains single, we recur 
   for f(n - 1)
2) n-th person pairs up with any of the 
   remaining n - 1 persons. We get (n - 1) * f(n - 2)

Therefore we can recursively write f(n) as:
f(n) = f(n - 1) + (n - 1) * f(n - 2)