Given the task definition and input, reply with output. You are given a math word problem and you are supposed to apply addition or subtraction mathematical operators on the numbers embedded in the text to answer the following question and then only report the final numerical answer.

Maria needs 21 cartons of berries to make a berry cobbler . She already has 4 cartons of strawberries and 8 cartons of blueberries . How many more cartons of berries should Maria buy ?
9