rotate the camera in the z axis

Hi, I’m looking for a little help.
as I can rotate the camera in the z axis by pressing the “horizontal” button?
eg:

to turn right, the z axis rotate from 0 ° to 25 °
to turn left, the z axis rotate from 0 ° to -25

It can be written better, sure.

using UnityEngine;
using System.Collections;

public class RotateZ : MonoBehaviour {

    public float speed = 10;

	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
        float s = Mathf.Sign(Vector3.Cross(transform.up, Vector3.up).z);
        float angle = Vector3.Angle(transform.up, Vector3.up);
        float ammount = Input.GetAxis("Horizontal") * Time.deltaTime * -speed;
            
        if(angle < 25 || s == Mathf.Sign(ammount)) transform.Rotate(0, 0, ammount);
	}
}