EXHIBIT A


                             JOINT FILING AGREEMENT
                             ----------------------

In accordance with Rule 13d-1(k)(l) under the Securities Exchange Act of 1934,
as amended, the undersigned hereby agree to the joint filing of the attached
Schedule 13G, and any and all amendments thereto, and expressly authorize
AMVESCAP PLC, as the ultimate parent company of each of its undersigned
subsidiaries, to file such Schedule 13G, and any and all amendments thereto, on
behalf of each of them.

Dated: May 9, 2007                     AMVESCAP PLC

                                       By:  /s/ Lisa Brinkley
                                          ----------------------------------
                                       Name:  Lisa Brinkley
                                       Title: Global Compliance Director


                                       PowerShares Capital Management LLC

                                       By:   /s/ Kevin Gustafson
                                          ----------------------------------
                                       Name:  Kevin Gustafson
                                       Title: General Counsel & COO