TDD Kata warm ups

Write a function (or functions) that given a collection of files will produce a sum of integers from each line of each file. 

	Each file can have any number of lines from 1 to N. 

	Each line can contain only one integer and no other alphanumeric characters. 

	All of the numbers from all of the files should be added to the final result.

	The result is just one number. 

The solution should come in a form of correct (compiling) code in the language of your choice and some notes. 

Review 

* What is the complexity of your solution?
* What design patterns (if any) you used?