Mingi's math test covers from page 8 to page 21 of the textbook. How many pages is it in total?
8 21 1 [OP_LIST_ARANGE] [OP_LIST_LEN]
var_a = 8
var_b = 21
var_c = 1
list_a = [i for i in range(var_a, var_b + 1, var_c)]
var_d = len(list_a)
print(int(var_d))