Keyword `void' cannot be used in this context

using UnityEngine; using System.Collections;

public class NeedleRotateAround : MonoBehaviour {

 public Transform gun;    
 public Transform pivot;    

 float prevAngle;
 
 
 void Start () 
 {
     prevAngle = gun.transform.rotation.eulerAngles.z;    
 }
 
 void Update () 
 {
     float currentAngle = gun.transform.rotation.eulerAngles.z;    

     float deltaAngle = currentAngle - prevAngle;    
     
     if(deltaAngle != 0)
         transform.RotateAround(pivot.position, Vector3.forward, deltaAngle);
     
     prevAngle = currentAngle;
 }

}

after typed those, console show me two errors. 1. Keyword void' cannot be used in this context 2. Unexpected symbol (', expecting )', ,', ;', [', or `='

it said these happened on line 16. why is this happened?