class Graph:
    def __init__(self):
        self.graph = {}
        
    def add_vertex(self, vertex):
        if vertex not in self.graph:
            self.graph[vertex] = []
        else:
            print(f"Vertex {vertex} already exists.")
            
    def add_edge(self, vertex1, vertex2, length:int):
        if vertex1 in self.graph and vertex2 in self.graph:
            self.graph[vertex1].append(vertex2)
            self.graph[vertex2].append(vertex1)
            self.length = length
        else:
            print("One or both vertices not found.")
    
    def display(self):
        for vertex in self.graph:
            print(f'{vertex}: {self.graph[vertex]} : {self.length}' )

# Example usage
if __name__ == "__main__":
    g = Graph()
    
    # Adding vertices
    g.add_vertex("A")
    g.add_vertex("B")
    g.add_vertex("C")
    g.add_vertex("D")
    
    # Adding edges
    g.add_edge("A", "B", 10)
    g.add_edge("A", "C", 5)
    g.add_edge("B", "D", 30)
    g.add_edge("C", "D", 11)
    
    # Display the graph
    g.display()