class Poligon:
    def __init__(self):
        self.__vertexs = []
        
    def __len__(self):
        return len(self.__vertexs)

    def __getitem__(self, i):
        return self.__vertexs[i]
    
    def afegir_vertex(self, p):
        self.__vertexs.append(p)
        
    def obtenir_aresta(self, i):
        if i+1 < len(self):
            return self.__vertexs[i], self.__vertexs[i+1]
        else:
            return self.__vertexs[-1], self.__vertexs[0]

    def iter_arestes(self):
        for i in range(len(self)):
            yield self.obtenir_aresta(i)