There's one less girl on Rihanna's "Last Girl on Earth" tour: Nicki Minaj just dropped out.

Rihanna & Nicki Minaj
Though the tour was only announced two days ago Minaj tweeted this afternoon: "I'm sorry 2 say I will not be going out on RiRi's tour ... I've decided 2 use that time 4 the completion of my album. Hope u understand."

Ke$ha will still be joining RiRi on the road.

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