Code:`we call s is a topology of a set S,if s is a set of several subsets of S which satisfied:`

1.s contains empty set and S;

2.if a and b belong to s ,then the intersection of a and b also belongs to s;

3.the unity of several sets which belong to s is also belongs to s.

assume a set S = {1,2,...,n} . How many topologies of S do we have?