C# Need to set a limit on rotation

How do I set a limit on how far I can rotate the arm? using UnityEngine; using System.Collections;

 public class ArmRotation : MonoBehaviour {
 
     public int rotationOffset = 90;
 
     // Update is called once per frame
     void Update () {
         // subtracting the position of the player from the mouse position
         Vector3 difference = Camera.main.ScreenToWorldPoint (Input.mousePosition) - transform.position;
         difference.Normalize ();        // normalizing the vector. Meaning that all the sum of the vector will be equal to 1
 
         float rotZ = Mathf.Atan2 (difference.y, difference.x) * Mathf.Rad2Deg;    // find the angle in degrees
         transform.rotation = Quaternion.Euler (0f, 0f, rotZ + rotationOffset);
     }
 }