Unity 5 FPS Camera

Hello, I'm a student that makes FPS Game.

I make FPS Game, but I have one problem.

I applied this script to Main Camera

 using UnityEngine;
 using System.Collections;
 
 public class Cameras : MonoBehaviour {
 
     Vector3 V3;
     
     public float TurnSpeed = 2.0f;
 
     // Use this for initialization
     void Start () { 
                            
     }
     
     // Update is called once per frame
     void Update () {
 
         V3 = new Vector3 (-Input.GetAxis("Mouse Y"), 0, 0);
         
         transform.Rotate(V3 * TurnSpeed);
 
            }
 }

but this script makes Main Camera rotate 360 degrees. I think I have to limit the Vectors, but I can't find it.

So, how can I limit the Vectors? Or Is there something else that better than this script?