Hi @sebcbon! Thanks for reaching out and being part of the community! There are many ways you could go about doing simple math in nodes.
We have the Summarize node but there are also a number of other ways to do simple mathematics in nodes.
http://community.n8n.io/t/math-how-to-in-a-node/28706/2
For your specific case, have you tried using a single Edit Fields (Set) node instead of that branching if statement and doing the math there with a conditional set in the same expression? Then you can also remove the Merge node.