Definition: 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.

Positive Example 1 -
Input: Last year , 90171 people were born in a country , and 16320 people immigrated to it . How many new people began living in the country last year ?
Output: 106491

Positive Example 2 -
Input: There are 27 pencils in the drawer . Nancy placed 45 pencils in the drawer . How many pencils are now there in total ?
Output: 72

Negative Example 1 -
Input: Alyssa picked 42 pears and Nancy picked 17 pears from the pear tree . How many pears were picked in all ?
Output: 42

Negative Example 2 -
Input: The town of Milburg has 5256 grown - ups and 2987 children . How many people live in Milburg ?
Output: 2987

Now complete the following example -
Input: At the schools book fair Sam bought 13 adventure books and 17 mystery books . If 15 of the books were used , how many new books did he buy ?
Output: 


Output: 15