What is the number of outstanding options whose exercise price are between $1.50 to $6.40?
{"$1.50 to $1.90": {"Number Outstanding": 32913, "Number Vested and Exercisable": 32913}, "2.86 to 6.40": {"Number Outstanding": 209126, "Number Vested and Exercisable": 209126}, "8.04 to 11.72": {"Number Outstanding": 498869, "Number Vested and Exercisable": 498869}, "12.20 to 15.06": {"Number Outstanding": 2227421, "Number Vested and Exercisable": 1587924}, "16.06 to 24.00": {"Number Outstanding": 1772062, "Number Vested and Exercisable": 560632}, "31.99 to 42.21": {"Number Outstanding": 847010, "Number Vested and Exercisable": 42839}, "55.10 to 79.25": {"Number Outstanding": 297341, "Number Vested and Exercisable": 12492}, "": {"Number Outstanding": 5884742, "Number Vested and Exercisable": 2944795}}
number_outstanding_150_to_190 = df["$1.50 to $1.90"]["Number Outstanding"]
number_outstanding_286_to_640 = df["2.86 to 6.40"]["Number Outstanding"]
answer = number_outstanding_150_to_190 + number_outstanding_286_to_640