Mouse Possition last and curent

I have an object that i want to rotate based on mouse position. My problem is that i can’t store last mouse position in order to do something like :

if(newMousePos > lastMousePos)  transform.Rotate(0, speed*Time.deltaTime, 0); 
                           else transform.Rotate(0, -speed*Time.deltaTime, 0); 

I want my object to rotate left if i drag my mouse from right to left and rotate right otherwise. Ideas ?

Try this code:

var lastpos = Vector3.zero;

function Start(){
    lastpos = Input.mousePosition;
}
function Update(){
    if(!Input.GetMouseButton(0)) return;
    var delta = Input.mousePosition - lastpos;

    transform.Rotate(0, speed * Time.deltaTime * delta.x, 0);
    lastpos = Input.mousePosition;
}