Access Main Camera and store it in a variable for quick access?

does anybody know how to store a reference of main camera into a variable without instantiating a new one?

my current code is

 using UnityEngine;
 using System.Collections;
 
 [ExecuteInEditMode]
 public class CameraSnap : MonoBehaviour {
 
     public int X = 0;
     public int Y = 0;
     public int Width = 160;
     public int Height = 144;
 
     private Camera cam = GameObject.GetComponent<UnityEngine.Camera>();
     private float res = Screen.width / Screen.height;
         
     // Update is called once per frame
     void Update () {
         cam.orthographic = true;
         cam.orthographicSize = res;
         cam.pixelRect = new Rect(Screen.width - X, Screen.height - Y, Width, Height);
 
     }
 }

but I get an error of "An object reference is required for the not-static field, method, or property 'Unity.GameObject.GetComponent()'" on line 12 on the cam declaration.