Limiting X-Axis rotation to 90 degrees and -90 degrees

So I am working on a very basic first-person shooter style game. However, my current character who happens to be a capsule is very happy rolling around X-axis and doing frontflips and backflips. I have been researching this a bit, but have been unable to find any code that works to limit this! Help would be appreciated!

Here's my code:

using UnityEngine; using System.Collections;

public class MoveWithMouse : MonoBehaviour {

 public float lookSensitivity = 5f;
 public float xRotation;
 public float yRotation;
 public float currentXRotation;
 public float currentYRotation;
 public float xRotationV;
 public float yRotationV;
 public float lookSmoothDamp;


 // Update is called once per frame
 void Update()
 {

     xRotation -= Input.GetAxis("Mouse Y") * lookSensitivity;
     yRotation += Input.GetAxis("Mouse X") * lookSensitivity;

     transform.rotation = Quaternion.Euler(xRotation, yRotation, 0);


     
    Cursor.visible = false;

     bool esc = Input.GetKey(KeyCode.Escape);

     if (esc) { Cursor.visible = true; }


 }
 

}