Detailed Instructions: Given a math word problem, answer the following question. You might need to apply addition or subtraction mathematical operators on the numbers embedded in the text to answer the question and then only report the final numerical answer.
Problem:a waiter at 'the greasy spoon ' restaurant had 39 customers to wait on . during the lunch rush he added another 12 customers . if 49 of the customers did n't leave him a tip how many customers did leave a tip ?
Solution:
2