Classical negation can be expressed by one of its halves

We present the logic K/2 which is a logic with classical implication and only the left part of classical negation. We show that it is possible to define a classical negation into K/2 and that the classical proposition logic K can be translated into this apparently weaker logic. We use concepts from model-theory in order to characterized rigorously this translation and to understand this paradox. Finally we point out that K/2 appears, following Haack's distinction, both as a deviation and an extension of K.