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; }



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