how to slow down the rotation from to ?

this code makes my object go so fast from the current y rot to the target

i using working on a third person camera code that's making my charcter follow the camera rot when i press the arrow key

but it's turning around so fast when i press the key and i don't know how to slow it down

 float rY = Mathf.SmoothStep(transform.eulerAngles.y,cam.transform.eulerAngles.y,1f - Time.deltaTime);


 transform.eulerAngles = new Vector3( transform.eulerAngles.x, rY, transform.eulerAngles.z );