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);

Here’s a quick search result on how to set rotation limits