2 power supply and 1 lamp puzzle help !

hello guys :
I have two Ac main lines.. and I have only one lamp.

I want to switch this lamp between this two ac source if any phase disappear..

eg : AC 1 line is available then the lamp should be on..
if ac 1 goes off then the lamp should automatically get connected to ac 2 line for power supply.

means the lamp switches between the main lines ( ac1 and ac2) if either power supply goes off.

Use a double pole-double throw DPDT relay, its coil rated and fed by AC line 1. Its common contacts C to the lamp, its normally closed contacts NC to AC line 2, its normally open contacts NO to AC line 1.