pre_text: The ESPP provides for annual increases in the number of shares available for issuance under the ESPP on the first day of each fiscal year beginning in fiscal 2014, equal to the least of: (i) 1% of the outstanding shares of all classes of common stock on the last day of the immediately preceding year; (ii) 1,250,000 shares; or (iii) such other amount as may be determined by the board of directors. During the year ended December 31, 2019, a total of 810,459 shares of Class A common stock were added to the ESPP Plan in connection with the annual increase provision. At December 31, 2019, a total of 3,918,712 shares were available for issuance under the ESPP. The weighted-average assumptions used to value ESPP rights under the Black-Scholes-Merton option-pricing model and the resulting offering grant date fair value of ESPP rights granted in the periods presented were as follows: As of December 31, 2019 and 2018, there was approximately $2.3 million and $1.5 million of unrecognized share-based compensation expense, net of estimated forfeitures, related to ESPP, which will be recognized on a straight-line basis over the remaining weighted-average vesting periods of approximately 0.4 years, respectively
table: Year ended December 31,
table: 2019 2018 2017
table: Expected term (in years) 0.5 0.5 0.5
table: Expected volatility 47% 42% 34%
table: Risk-free interest rate 2.01% 2.31% 1.20%
table: Expected dividend yield 0% 0% 0%
table: Offering grant date fair value of ESPP rights $33.66 $18.07 $9.52
question: Analyse this data from a financial earnings document. What is the average offering grant date fair value of ESPP rights between 2017 to 2019?