12V o/p for 100-200V input

I want to make a circuit which generates 12 volt when input is 100 volts DC minimum and the o/p should remain constant 12V for maximum input of 200 Volts.
Using 7812 how do i step down the input voltage within permissible values.
Please need urgent help.

It isn't possible with an LM7812, you need a switching regulator.