Without taking into account the share-based compensation expenses, our general and administrative expenses have decreased in absolute dollar terms due to the decrease of our travelling expenses caused by the COVID-19 breaking in the first half year of 2020 and the decrease of consulting fee as the cooperation with several service providers have been terminated. We expect our general and administrative expenses will increase in absolute dollar terms in the future, in line with the expansion of our network business and hospital business and the growth in our total net revenues as well as the well-controlling of COVID-19 in the PRC.
Impairment of Long-lived Assets. Our impairment of long-lived assets was RMB5.4 million, RMB76.1 million and RMB8.5 million (US$1.3 million) for the years ended December 31, 2018, 2019 and 2020, respectively.
Share-based Compensation
On October 16, 2008, our board of directors adopted the 2008 share incentive plan. The plan provided for the grant of options, share appreciation rights, or other share-based awards to key employees, directors or consultants. Our board of directors and shareholders initially authorized the issuance of up to 4,765,800 ordinary shares upon exercise of awards granted under our 2008 share incentive plan. On November 26, 2011, our board of directors and the shareholders authorized the issuance of additional 5,101,968 ordinary shares under the 2008 share incentive plan. On May 29, 2015, our board of directors and the shareholders authorized the issuance of additional 4,940,550 ordinary shares under the 2008 share incentive plan.
On November 27, 2009 and September 30, 2011, we granted options to purchase a total of 4,765,800 ordinary shares at exercise prices of US$3.67 and US$2.17 per share, respectively, under our 2008 share incentive plan to our directors and employees. On February 18, 2014, we granted options to purchase 3,479,604 shares at an exercise price of US$2.04 per share that have a contractual life of eight years and vest over four equal installments on the first, second, third and fourth anniversary of the grant date. We also granted 1,370,250 restricted shares, 21,132 restricted shares and 69,564 restricted shares on February 18, 2014, July 1, 2014 and August 1, 2014, respectively, to certain directors, officers and employees. On August 7, 2017, August 8, 2017 and September 13, 2017, we granted 1,453,950 restricted shares, 3,319,200 restricted shares and 45,000 restricted shares, respectively, to certain directors, officers and employees. On October 2, 2018, we granted 5,992,605 restricted shares to certain directors, officers and employees. The restricted shares vest over four equal installments on the first, second, third, and fourth anniversary of the grant date.
We recognize the compensation expense on a straight-line basis over the requisite service period for the entire award. With respect to share options, we calculated the estimated grant date fair value of the share options granted on the date of grant, using a Binomial Tree Model. The risk-free rate was based on the US Treasury bond yield curve in effect at the time of grant for periods corresponding with the expected term of the option. The dividend yield was estimated based on the average of our historical dividend yields. The volatility assumption was estimated based on the historical price volatility of ordinary shares of comparable companies in the health care industry. The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying awards and the fair value of the our shares that would have been received by the option holders if all in-the-money options had been exercised on the issuance date.
We recorded share-based compensation expenses of approximately RMB11.1 million in 2018, RMB20.6 million in 2019 and RMB20.6 million (US$3.2 million) in 2020. The 2008 share incentive plan was terminated on the tenth anniversary of the effective date in October 2018. The awards granted prior to the termination date are still subject to the 2008 share incentive plan.
Taxation
Cayman Islands
We are incorporated in the Cayman Islands. Under the current laws of the Cayman Islands, we are not subject to income or capital gains tax. In addition, dividend payments made by us are not subject to withholding tax in the Cayman Islands.
British Virgin Islands
Certain of our subsidiaries are established in the British Virgin Islands and under the current laws of the British Virgin Islands, such subsidiaries are not subject to income or capital gains tax.