{"pre_text": ["humana inc .", "notes to consolidated financial statements 2014 ( continued ) value , or the excess of the market value over the exercise or purchase price , of stock options exercised and restricted stock awards vested during the period .", "the actual tax benefit realized for the deductions taken on our tax returns from option exercises and restricted stock vesting totaled $ 16.3 million in 2009 , $ 16.9 million in 2008 , and $ 48.0 million in 2007 .", "there was no capitalized stock-based compensation expense .", "the stock plans provide that one restricted share is equivalent to 1.7 stock options .", "at december 31 , 2009 , there were 12818855 shares reserved for stock award plans , including 4797304 shares of common stock available for future grants assuming all stock options or 2821944 shares available for future grants assuming all restricted shares .", "stock options stock options are granted with an exercise price equal to the average market value of the underlying common stock on the date of grant .", "our stock plans , as approved by the board of directors and stockholders , define average market value as the average of the highest and lowest stock prices reported by the new york stock exchange on a given date .", "exercise provisions vary , but most options vest in whole or in part 1 to 3 years after grant and expire 7 to 10 years after grant .", "upon grant , stock options are assigned a fair value based on the black-scholes valuation model .", "compensation expense is recognized on a straight-line basis over the total requisite service period , generally the total vesting period , for the entire award .", "for stock options granted on or after january 1 , 2010 to retirement eligible employees , the compensation expense is recognized on a straight-line basis over the shorter of the requisite service period or the period from the date of grant to an employee 2019s eligible retirement date .", "the weighted-average fair value of each option granted during 2009 , 2008 , and 2007 is provided below .", "the fair value was estimated on the date of grant using the black-scholes pricing model with the weighted-average assumptions indicated below: ."], "post_text": ["when valuing employee stock options , we stratify the employee population into three homogenous groups that historically have exhibited similar exercise behaviors .", "these groups are executive officers , directors , and all other employees .", "we value the stock options based on the unique assumptions for each of these employee groups .", "we calculate the expected term for our employee stock options based on historical employee exercise behavior and base the risk-free interest rate on a traded zero-coupon u.s .", "treasury bond with a term substantially equal to the option 2019s expected term .", "the volatility used to value employee stock options is based on historical volatility .", "we calculate historical volatility using a simple-average calculation methodology based on daily price intervals as measured over the expected term of the option. ."], "table": [["", "2009", "2008", "2007"], ["weighted-average fair value at grant date", "$ 14.24", "$ 17.95", "$ 21.07"], ["expected option life ( years )", "4.6", "5.1", "4.8"], ["expected volatility", "39.2% ( 39.2 % )", "28.2% ( 28.2 % )", "28.9% ( 28.9 % )"], ["risk-free interest rate at grant date", "1.9% ( 1.9 % )", "2.9% ( 2.9 % )", "4.5% ( 4.5 % )"], ["dividend yield", "none", "none", "none"]], "table_ori": [["", "2009", "2008", "2007"], ["Weighted-average fair value at grant date", "$14.24", "$17.95", "$21.07"], ["Expected option life (years)", "4.6", "5.1", "4.8"], ["Expected volatility", "39.2%", "28.2%", "28.9%"], ["Risk-free interest rate at grant date", "1.9%", "2.9%", "4.5%"], ["Dividend yield", "None", "None", "None"]]}

Question: what was the percent of the change of the weighted-average fair value at grant date from 2008 to 2009
Evidence:
table_1: the weighted-average fair value at grant date of 2009 is $ 14.24 ; the weighted-average fair value at grant date of 2008 is $ 17.95 ; the weighted-average fair value at grant date of 2007 is $ 21.07 ;
Program: subtract(14.24, 17.95), divide(#0, 17.95)
Answer:
str_answer: -21%
exe_answer: -0.20669