How do you enter the Tallest biulding in Saffron city

go to the game corner in celadon city talk to the rocket then talk to the poster and press yes then go down the stairs and get to the end and fight Giovanni the team rocket boss
lv 22 onix
lv 29 kangaskan
lv 22 ryhorn
then go to lavander town and beat team rocket there and then it will be open