Name that node #1 (Beginner's Level Puzzle)

While Puzzling.SE has some extremely clever and difficult puzzles that are being solved by people whose intellect amazes me, I wanted to make some graph puzzles that were specifically meant for complete beginners. This puzzle is a test run for this type. If you're looking at this puzzle and think it is extremely trivial, please leave it for someone else to solve. If you find yourself spending mental effort to understand this puzzle, then it is meant for you.

What is the value of the node labeled ?, and what is the rule that determines its value?

Beginner tip: Remember to use >! to hide the key details of your answer so that it doesn't spoil it for others.

Hint 1

The key to understanding this puzzle is two understand the relationship between the numbers in the nodes and the numbers along edges.

For example $$1+10=11$$.
Therefore $$40-19=21$$.