Check if GetMouseButtonDown hits UI first

gistfile1.cs

EventSystem eventSystem;
//..
void Start () 
{
	GameObject go = GameObject.Find("EventSystem");
	if (go==null)
	{
		Debug.LogError("EventSystem not founded..");
	}else{
		eventSystem = go.GetComponent<UnityEngine.EventSystems.EventSystem>();
	}
}
// ..
void Update () 
{
	if (Input.GetMouseButtonDown(0)) 
	{
		if (eventSystem.IsPointerOverGameObject()) return;
		
		// do other stuff, since not hitting UI
	}