Sum Numbers in a File

A text file contains whole numbers separated by spaces or newlines, for example:

10
20
30

Write a function int sumFile(const string& filename) that returns the sum of every number in the file. The starter opens the file and already loops with while (in >> n); add the line inside the loop that adds n to total. Numbers may be negative and several may share one line.

The test prints the returned sum on its own line, for example (see Expected Output).

Do NOT write a main function.