Chess puzzle (Advantage)
Opening: Queens_Pawn_Game, Queens_Pawn_Game_Mason_Variation
Moves: 1. d4 d5 2. Bf4 Nf6 3. e3 e6 4. Nd2 Bd6 5. c3 c5 6. Bg3 Nc6 7. Bd3 cxd4 8. exd4
Bxg3 9. hxg3 h6 10. Ngf3 Bd7 11. Qe2 Qc7 12. Ne5 Nxe5 13. dxe5 Ng8 14. f4 O-O-O
15. Nf3 Ne7 16. Nd4 Kb8 17. a4 Nc6 18. Nb5 Qb6 19. b4 a6 *
Position (FEN): 1k1r3r/1p1b1pp1/pqn1p2p/1N1pP3/PP3P2/2PB2P1/4Q1P1/R3K2R w KQ - 0 20
White to move.
Themes:  Middlegame  
Puzzle rating: 1461
Best move: a5