Missed Chance (1)

In the diagram below you see a position from Howell - Carlsen (London 2009). Black to move missed a simple win.

How it could have gone: 52.- Ra2+ 53.Kg3 (53.Kh3 Rh1+) Rg1+ 54.Kf4 Rf2+ 55.Kxe4 Rxg4+ 56.Kd3 Rxf5 +-

Since Carlsen didn't play this, the game ended drawin move 79.