LONDON -- Usain Bolt has a new challenger, and maybe even a new distance to think about.

The biggest star in track is being sought out by Mo Farah for a charity race somewhere between Bolt's sprint specialty and Farah's distance domain.

"It'd be great to be able to do a distance where people vote in what distance will be suitable, and then get a judge and then come in the middle with that distance and train for it," said Farah, a British runner who won the 5,000 and 10,000 metres at last year's London's Olympics. "Bolt, are you up for that? Come on, you got to do it."

Bolt has little left to prove on the track. The Jamaican won the 100 and 200 in world record time at the 2008 Beijing Games, and then became the first man to defend those titles at last year's Olympics. He also helped Jamaica win the 4x100 relays at both.

On Friday in London, Bolt won the 100 metres at the Anniversary Games, a Diamond League meet that took place in the Olympic Stadium to mark one year since the start of the 2012 Olympics. Farah won the 3,000 on Saturday.

Despite the differences in distances, Bolt seems to be considering Farah's challenge.

"That sounds fun. It's going to be hard, but for me it's charity, so it's just all about fun and enjoyment," Bolt said. "For me, I'm up for anything if it's possible."

The hard part would be deciding what distance to race.

Bolt may be the best sprinter of all time, and has often been rumoured to be adding the 400 to his repertoire. Farah, however, is a distance specialist who is talking more and more about moving in the other direction and adding the marathon to his running regime.

So even one of the middle distances, like the 1,500 metres, would be hard to see coming.

"Way too far," Bolt said. "Six-hundred for sure I can try because I've done 600 in training."

Two running stars meeting at a made-up distance is not new. Nearly a year after the 1996 Olympics, 200-400 gold medallist Michael Johnson and 100 champion Donovan Bailey met in a much-hyped 150-meter race in Toronto.

About halfway through, Johnson pulled up with a muscle injury in his left thigh, leaving Bailey to win $1 million.