How can I add inertia to a rotation when I release the mouse?

I have an sphere, and I want to rotate it when I drag my mouse, but it stops when I release the mouse. I pretend to continue the rotation after the dragging, slowing down the movement until it stops (inertia). How can I do that? I have read that I should save the last known rotation speed, and decrease that speed until 0, but I don´t know how can I implement it to my code.

(Here is the rotation code, in void Update())

   if (cameraDragging)
         {
 
             if (Input.GetMouseButtonDown(0))
             {
                 dragOrigin = Input.mousePosition;
                 return;
             }
 
             if (!Input.GetMouseButton(0)) return;
 
             Vector3 pos = Camera.main.ScreenToViewportPoint(Input.mousePosition - dragOrigin);
                 Vector3 move = new Vector3(-pos.x * dragSpeed, -pos.y * dragSpeed, 0);
                     world.transform.Rotate(move.y , 0, move.x);       
         }

Thank you.