[a] total number of shares purchased during the quarter includes approximately 563220 shares delivered or attested to upc by employees to pay stock option exercise prices , satisfy excess tax withholding obligations for stock option exercises or vesting of retention units , and pay withholding obligations for vesting of retention shares . [b] on may 1 , 2008 , our board of directors authorized us to repurchase up to 40 million shares of our common stock through march 31 , 2011