In how many ways can we put n different letters into n different envelopes so that ALL the letters go into the wrong envelopes?

e.g.

Envelope: e1, e2 Letters: l1, l2

l2 into e1 and l1 into e2:

Ans = 1

For 3:

e1 into l2, e2 into l3, e3 into l1 - 1 way

e1 into l3, e2 into l1, e3 into l2 - 1 more way

Ans = 2

Remember ALL letters should go into wrong envelop