Rook ending

() vs. ()
- -

[Event "N1"] [Site "France"] [Date "2010.11.13"] [Round "?"] [White "GL"] [Black "SL"] [Result "*"] [WhiteElo "2152"] [BlackElo "2196"] [Annotator "arna"] [SetUp "1"] [FEN "8/5p1p/1k4p1/4K2P/R4PP1/pr6/8/8 b - - 0 0"] [PlyCount "23"] 1... gxh5 2. gxh5 Rb5+ 3. Kf6 Ra5 4. Rb4+ Kc5 5. Rb1 a2 6. Ra1 Kd4 7. h6 Ke4 8. f5 $1 (8. Kg7 Kf5 9. Kxh7 Kf6 10. Kg8 Ra8+ 11. Kh7 Ra7 12. Kg8 Kg6 13. Rg1+ Kxh6) 8... Ra6+ $1 9. Kg7 (9. Kxf7 Kxf5) 9... f6 10. Kxh7 (10. Rf1 a1=Q 11. Rxa1 Rxa1 12. Kxf6 Ra6+ 13. Kg5 Kf3 14. f6 Ra5+ 15. Kh4 Rf5) 10... Kxf5 11. Kg7 Ra7+ 12. Kf8 Kg6 {black wins} *

"Must win" situation. How should black proceed ?