Need help with C# error "No Overload for method "Box" takes 1 argument"

Hello i'm a newbie to C# so i was wonder if any one could help me here I'm trying to make a simple health bar/ hit point bar which ever one you want to call it but it keeps coming up with "No Overload for method 'Box; takes '1' arguments"
Heres the sample of how I've done it. Thanks

Code:

using UnityEngine;
using System.Collections;

public class playerHealth : MonoBehaviour {
	public int maxHealth = 100;
	public int curHealth = 100;

	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
	
	}
	
	void OnGUI() {
		GUI.Box(new Rect(10, 10, Screen.width / 2 / (maxHealth / curHealth), 20) + "/" + maxHealth);
    }
}