. the weighted average grant-date fair value of each designated share purchased under this plan during the years ended may 31 , 2013 , 2012 and 2011 was $ 6 , $ 7 and $ 6 , respectively , which represents the fair value of the 15% ( 15 % ) discount . stock options stock options are granted at 100% ( 100 % ) of fair market value on the date of grant and have 10-year terms . stock options granted vest one year after the date of grant in 25% ( 25 % ) increments over a four year period