How salty is olive brine?
There is no single answer to this. Olives are packed in widely differing brine concentrations, and breads are made with widely different salt concentrations. If you start reading the labels of olives in the supermarket, you will find anything between 1.5 and 7-8% salt.
Luckily, the labels are also your solution here. Just read the label, assume that the olives have reached equilibrium with the brine (this is not too strong an assumption, if you decide to desalt olives at home, you will find that it works within 1-2 days) and calculate from there.