You could use Camera.ScreenToViewportPoint https://docs.unity3d.com/ScriptReference/Camera.ScreenToViewportPoint.html

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
   void Update() {
       transform.position = Camera.main.ScreenToViewportPoint(Input.mousePosition);
   }
}

You could use Camera.ScreenToViewportPoint https://docs.unity3d.com/ScriptReference/Camera.ScreenToViewportPoint.html

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
   void Update() {
       transform.position = Camera.main.ScreenToViewportPoint(Input.mousePosition);
   }
}

I could change your code as this.

void Update()
{
    Ray ray = cam.ViewportPointToRay(cam.ScreenToViewportPoint(Input.mousePosition));
    RaycastHit hit;
    ...
}