snap to 180 or -180 degrees on going left and right

I have looked all over but I can't figure this out. I want it so when I walk to the left by pressing A, it snaps my character to 180 degrees on the y axis and when I walk right when I press D, it snaps to 0 degrees. This is what I have, but it rotates over time and continues past my desired angles: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class Rotate : MonoBehaviour { float speed =6f;

 // Use this for initialization
 void Start () {
 }
 
 // Update is called once per frame
 void Update () {
     {

             if (Input.GetKey(KeyCode.A))
                 transform.Rotate(Vector3.up * speed * Time.deltaTime);

             if (Input.GetKey(KeyCode.D))
                 transform.Rotate(-Vector3.up * speed * Time.deltaTime);

         }
         }
     }