EXHIBIT 2

JOINT FILING AGREEMENT

The undersigned hereby agree to the joint filing of the Amendment No. 5 to Schedule 13D to which this Agreement is attached.

Dated:  May 18, 2011

DISCOVERY GROUP I, LLC

for itself and as general partner of

DISCOVERY EQUITY PARTNERS, L.P.

By Michael R. Murphy*                                                   

      Michael R. Murphy

      Managing Member

Daniel J. Donoghue*                                                       

Daniel J. Donoghue

Michael R. Murphy*                                                       

Michael R. Murphy

*By: /s/ Mark Buckley                                                     

          Mark Buckley

         Attorney-in-Fact for Daniel J. Donoghue

         Attorney-in-Fact for Michael R. Murphy