There are 18 boxes that can fit 15 masks. If there are 3 masks lacking in each box, what is the total number of masks?
15 3 [OP_SUB] 18 [OP_MUL]
var_a = 15
var_b = 3
var_c = var_a - var_b
var_d = 18
var_e = var_c * var_d
print(int(var_e))