Make a cube rotate exactly 90 degrees rather than keeping it rotating

I am very new to unit.I am trying to make a geometry dash like game but cant seem to make the cube rotate 90 degrees when jumping.It either keeps rotating or it rotate 8 degrees.Any response would be helpful but if you can put a not so complicated code would be even more welcomed.

UPDATE : I made it rotate but for some reason it rotates once to the left and then then next jump it rotates to the right

// using System.Collections; using System.Collections.Generic; using UnityEngine; public class Player : MonoBehaviour {

 bool isGrounded = false;
 public float jumpForce;
 public Rigidbody2D rb;
 public GameObject player;
 public GameObject sprite;
 public float speed = 55f;
 public float rotation = 0f;
 private Quaternion qTo;
 void Start() {
     rb = GetComponent<Rigidbody2D>();
 }

 void Update()
 {
     if (player.transform.position.x < -7.1)
     {
         Destroy(player);
     }

     if (isGrounded == false)
     {
         qTo = Quaternion.Euler(0, 0, rotation);
         sprite.transform.rotation = Quaternion.Slerp(sprite.transform.rotation, qTo, 0.15f);
     }
 }

 void FixedUpdate()
 {
     if (Input.GetKey(KeyCode.Space))
         if (isGrounded == true)
         {
             rb.velocity = new Vector2(0, jumpForce * rb.mass * rb.gravityScale);
         }
 }

 void OnCollisionEnter2D(Collision2D other)
 {
     if (other.collider.tag == "Ground")
         isGrounded = true;
     
 }

 void OnCollisionStay2D(Collision2D other)
 {
     if (other.collider.tag == "Ground")
     {
         isGrounded = true;
     } 
 }

 void OnCollisionExit2D(Collision2D other)
 {
     if (other.collider.tag == "Ground")
     {
         isGrounded = false;
     }
     if (Input.GetKey(KeyCode.Space))
     {
         rotation -= 90f;
     }
 }

}