Britney Spears 19, in Brazil for the all-star Rock in Rio music festival, declared on Friday that she and Madonna, 42, are definitely going to make music together. “The duet with Madonna is going to happen,” Spears told the local press in Latin America, a spokeswoman for Virgin Records informed Reuters. “Madonna is great. I grew up listening to her in my room . . . I was really happy when she said she admired me.” The two pop divas from two separate generations have been running a mutual admiration society of late, with both expressing their appreciation of one another. Madonna has even publicly sported a Britney T-shirt. “Since we’re both busy, we have to work out the best schedule to meet at the studio,” the “Oops . . . I Did It Again!” girl said just hours before taking to the stage before some 200,000 reportedly screaming fans in Rio de Janeiro. Reuters noted that there has been no immediate comment from the Madonna camp on Britney’s announcement.