How to set memento in memento pattern

Question

I need to write a Graph class which performs some sort of operations and gives possibility to save it using memento pattern. I am trying to implement that functionality, but I want to have IGraph interface which cannot return explicit type of ConcreteMemento so it returns Memento interface, but because of that I need to downcast it inside Graph class.

So my question is, is it a good practise or am I missing something in memento pattern? Also is it possible to implement ConcreteMemento in the way that only Graph can access some of its methods?

Grahp

    class ConcreteMemento : Memento
    {

    }
    internal class Graph : IGraph
    {
        public Memento GetMemento()
        {
            return new ConcreteMemento();
        }

        public void SetMemento(Memento m)
        {
            var concreteMemento = m as ConcreteMemento;
        }
    }

IGraph

public interface Memento
    {

    }
    interface IGraph
    {
        Memento GetMemento();

    }
    ```