What was the change in Common stock subject to repurchase between 2018 and 2017?
{"Stock options": {"2019": 223259, "2018": 229294, "2017": 258917}, "Restricted stock awards": {"2019": "\u2014", "2018": "\u2014", "2017": 129}, "Restricted stock units": {"2019": 136600, "2018": 148175, "2017": 188050}, "Common stock subject to repurchase": {"2019": 250, "2018": 957, "2017": 13281}}
common_stock_subject_to_repurchase_2018 = df["Common stock subject to repurchase"]["2018"]
common_stock_subject_to_repurchase_2017 = df["Common stock subject to repurchase"]["2017"]
answer = common_stock_subject_to_repurchase_2018 - common_stock_subject_to_repurchase_2017