Context-oriented programming (COP) is a new programming approach whereby the context in which expressions evaluate can be adapted as a program runs. COP provides a degree of flexibility beyond object-oriented programming, while arguably retaining more modularity and structure than aspect-oriented programming. Although many languages exploring the context-oriented approach exist, to our knowledge no formal type-sound dynamic semantics of these languages exists. We address this shortcoming by providing a concise syntax-based formal semantics for context-oriented programming with layers, as witnessed by ContextL, ContextJ*, and other languages. Our language is based on Featherweight Java extended with layers and scoped layer activation and deactivation. As layers may introduce methods not appearing in classes, we also give a static type system that ensures that no program gets stuck (i.e., there exists a binding for each dispatched method call).