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.
Q: Context: Dave had 19 apps and 18 files on his phone. After deleting some apps and files he had 6 apps and 15 files left. 

 Question: How many more files than apps does he have left on the phone?
A: 
9