bhge 2018 form 10-k | 85 it is expected that the amount of unrecognized tax benefits will change in the next twelve months due to expiring statutes , audit activity , tax payments , and competent authority proceedings related to transfer pricing or final decisions in matters that are the subject of litigation in various taxing jurisdictions in which we operate . at december 31 , 2018 , we had approximately $ 96 million of tax liabilities , net of $ 1 million of tax assets , related to uncertain tax positions , each of which are individually insignificant , and each of which are reasonably possible of being settled within the next twelve months . we conduct business in more than 120 countries and are subject to income taxes in most taxing jurisdictions in which we operate . all internal revenue service examinations have been completed and closed through year end 2015 for the most significant u.s . returns . we believe there are no other jurisdictions in which the outcome of unresolved issues or claims is likely to be material to our results of operations , financial position or cash flows . we further believe that we have made adequate provision for all income tax uncertainties . note 13 . stock-based compensation in july 2017 , we adopted the bhge 2017 long-term incentive plan ( lti plan ) under which we may grant stock options and other equity-based awards to employees and non-employee directors providing services to the company and our subsidiaries . a total of up to 57.4 million shares of class a common stock are authorized for issuance pursuant to awards granted under the lti plan over its term which expires on the date of the annual meeting of the company in 2027 . a total of 46.2 million shares of class a common stock are available for issuance as of december 31 , 2018 . stock-based compensation cost was $ 121 million and $ 37 million in 2018 and 2017 , respectively . stock-based compensation cost is measured at the date of grant based on the calculated fair value of the award and is generally recognized on a straight-line basis over the vesting period of the equity grant . the compensation cost is determined based on awards ultimately expected to vest ; therefore , we have reduced the cost for estimated forfeitures based on historical forfeiture rates . forfeitures are estimated at the time of grant and revised , if necessary , in subsequent periods to reflect actual forfeitures . there were no stock-based compensation costs capitalized as the amounts were not material . stock options we may grant stock options to our officers , directors and key employees . stock options generally vest in equal amounts over a three-year vesting period provided that the employee has remained continuously employed by the company through such vesting date . the fair value of each stock option granted is estimated using the black- scholes option pricing model . the following table presents the weighted average assumptions used in the option pricing model for options granted under the lti plan . the expected life of the options represents the period of time the options are expected to be outstanding . the expected life is based on a simple average of the vesting term and original contractual term of the awards . the expected volatility is based on the historical volatility of our five main competitors over a six year period . the risk-free interest rate is based on the observed u.s . treasury yield curve in effect at the time the options were granted . the dividend yield is based on a five year history of dividend payouts in baker hughes. .

2018: expected life ( years ): 6
risk-free interest rate: -2.5
volatility: -33.7
dividend yield: -2
weighted average fair value per share at grant date: 10.34
2017: expected life ( years ): 6
risk-free interest rate: -2.1
volatility: -36.4
dividend yield: -1.2
weighted average fair value per share at grant date: 12.32

baker hughes , a ge company notes to consolidated and combined financial statements .

Questions:
what is the total number of shares authorized for sales less those available for sale?
how many shares are available for sale?
what is the difference over the available shares?

Answers:
11.2
57.4
19.5%