Given the task definition, example input & output, solve the new input case.
You are given a math word problem and you are supposed to only use subtraction on the numbers embedded in the text to answer the following question and then only report the final numerical answer.
Example: Context: Sam is 6 feet tall and Alex is 5 feet tall 

 Question: By how many feet is Sam taller than Alex? 
Output: 1
 Sam's height - Alex's height = 6-5 = 1

New input case for you: Context: Paul had 50 books. After buying some in a garage sale he had 151 left. 

 Question: How many books did he buy?
Output:
101