Script Errors (CSO119, CS1503 & CS1503)

The code is:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 public class Inventory : MonoBehaviour {
 
     public int slotX =5;
     public int slotY = 5;
     public GUISkin skin;
     public List <Item> inventory = new List<Item>();
     public bool showInventory = false;
 
     private Item swappedItem;
     private bool draggingItem = false;
     private Item draggedItem;
     private int prevIndex;
     private int spikeCount;
     private Rect invRect = new Rect(200,140,350,420);
     private Rect imageRect = new Rect(0,0,50,50);
 
     // Use this for initialization
     void Start () {
         invRect = new Rect (Screen.width / 2(invRect.width / 2), 0 , invRect.width , invRect.height );
 
         for(int i=0; i < slotX * slotY; i++)
         {
 
             inventory.Add (new Item());
         }
     }
     
     // Update is called once per frame
     void Update () {
         if(Input.GetButtonDown("Inventory"))
            {
             showInventory = true;
 
             if(draggingItem){
 
                 inventory[prevIndex] = draggedItem;
                 draggingItem = false;
                 draggedItem = null;
 
             }
         }
     
     //Cheat keys
         if (Input.GetKeyDown("1"))
         {   
             AddItem("Wood");
         }
 
         if (Input.GetKeyDown("2")){
             AddItem("Stone");
         }
     
     }
 }

Errors are:

CSO119: Expression denotes a 'value', where a 'method group' was expected.

CS1503 :Argument #1 cannot convert object to type float.

CS1502: The best overloaded method match for 'UnitEngine.Rect.Rect(float,float,float,float)', has some invalid arguments.

All Are on line 16. Help!