Didn't use to = used not to = usedn't to ?

I prefer( used not to )

I suppose you do not confuse used to do sth with be used to do sth.
In informal English the negative form is didn't use to
In formal English it is used not to
But the contraction usedn't to sounds odd to me. Actually, I've never heard it. Maybe it was correct at some time in history, but I think it's wrong now. So forget about usedn't to and just use the other forms.

don't confuse (uesd to do something)and (be used to doing)

well,yea it is right but the usedn`t is nooooooooooooooooooot useful