Event B is supported by the RODIN platform and provides a framework for teaching programming methodology based on the famous pre/post specifications, together with the refinement. We illustrate a methodology based on Event B and the refinement by developing Floyd's algorithm for computing the shortest distances of a graph, which is based on an algorithm design technique called dynamic programming. The development is based on a paradigm identifying a non-deterministic event with a procedure call and by introducing control states. We discuss points related to our lectures at the university.