Chess puzzle (Advantage)
Opening: Four_Knights_Game, Four_Knights_Game_Double_Spanish
Moves: 1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. Bb5 Bb4 5. a3 a6 *
Position (FEN): r1bqk2r/1ppp1ppp/p1n2n2/1B2p3/1b2P3/P1N2N2/1PPP1PPP/R1BQK2R w KQkq - 0 6
White to move.
Themes:  Opening  
Puzzle rating: 1490
Best move: axb4