pre_text: Restricted Stock Units RSU activity is summarized as follows (shares in thousands): The weighted-average grant date fair value of RSUs granted during the years ended December 31, 2019, 2018, and 2017 was $55.69, $46.17, and $37.99, respectively. The total fair value of RSUs vested as of the vesting dates during the years ended December 31, 2019, 2018, and 2017 was $58.4 million, $49.9 million, and $37.2 million, respectively. Unrecognized compensation expense related to unvested RSUs was $127.2 million at December 31, 2019, which is expected to be recognized over a weighted-average period of 2.6 years.
table: Number of Shares Weighted- Average Grant Date Fair Value
table: Unvested shares at December 31, 2018 4,117 $41.94
table: Granted 1,589 55.69
table: Forfeited (510) 45.72
table: Vested (1,440) 40.61
table: Unvested shares at December 31, 2019 3,756 $47.76
question: Analyse this data from a financial earnings document. What is the difference in weighted-average grant date fair value for unvested shares in 2018 and vested shares?