performance share awards the vesting of psas is contingent upon meeting various individual , divisional or company-wide performance conditions , including revenue generation or growth in revenue , pretax income or earnings per share over a one- to five-year period . the performance conditions are not considered in the determination of the grant date fair value for these awards . the fair value of psas is based upon the market price of the aon common stock at the date of grant . compensation expense is recognized over the performance period , and in certain cases an additional vesting period , based on management 2019s estimate of the number of units expected to vest . compensation expense is adjusted to reflect the actual number of shares issued at the end of the programs . the actual issuance of shares may range from 0-200% ( 0-200 % ) of the target number of psas granted , based on the plan . dividend equivalents are not paid on psas . information regarding psas granted during the years ended december 31 , 2011 , 2010 and 2009 follows ( shares in thousands , dollars in millions , except fair value ) : .
what is the lowest value of unamortized expense during this period?
( 1 ) represents per share weighted average fair value of award at date of grant . during 2011 , the company issued approximately 1.2 million shares in connection with the 2008 leadership performance plan ( 2018 2018lpp 2019 2019 ) cycle and 0.3 million shares related to a 2006 performance plan . during 2010 , the company issued approximately 1.6 million shares in connection with the completion of the 2007 lpp cycle and 84000 shares related to other performance plans . stock options options to purchase common stock are granted to certain employees at fair value on the date of grant . commencing in 2010 , the company ceased granting new stock options with the exception of historical contractual commitments . generally , employees are required to complete two continuous years of service before the options begin to vest in increments until the completion of a 4-year period of continuous employment , although a number of options were granted that require five continuous years of service before the options are fully vested . options issued under the lpp program vest ratable over 3 years with a six year term . the maximum contractual term on stock options is generally ten years from the date of grant . aon uses a lattice-binomial option-pricing model to value stock options . lattice-based option valuation models use a range of assumptions over the expected term of the options . expected volatilities are based on the average of the historical volatility of aon 2019s stock price and the implied volatility of traded options and aon 2019s stock . the valuation model stratifies employees between those receiving lpp options , special stock plan ( 2018 2018ssp 2019 2019 ) options , and all other option grants . the company believes that this stratification better represents prospective stock option exercise patterns . the expected dividend yield assumption is based on the company 2019s historical and expected future dividend rate . the risk-free rate for periods within the contractual life of the option is based on the u.s . treasury yield curve in effect at the time of grant . the expected life of employee stock options represents the weighted-average period stock options are expected to remain outstanding and is a derived output of the lattice-binomial model. .