Madonna (entertainer):

The MDNA Tour, which further promoted the album, began in May 2012 in Tel Aviv, Israel. The tour has received positive critical reception, but featured controversial subjects such as violence, firearms, human rights, nudity and politics. Lawsuits threatened against Madonna have also been engaged from the tour. It was a box office success with a gross of $305.2 million from 88 sold-out shows, and became the highest-grossing tour of 2012 and the tenth highest-grossing tour of all time. At the 2013 Billboard Music Awards, Madonna won three trophies for Top Touring Artist, Top Dance Artist and Top Dance Album. Madonna was named the top-earning celebrity of the year by Forbes, earning an estimated $125 million, due to the success of the tour.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Who named Madonna as the top-earning celebrity of the year?
Forbes