Что за ошибка only assignment call increment decrement?

using UnityEngine;
using UnityEngine.UI;

public class gamescript : MonoBehaviour
{
 public static int money;
 public static int upgrades;

public void LoadSave() 
{
 money=PlayerPrefs.GetInt("money");
 upgrades=PlayerPrefs.GetInt("upgrades");
}

public void click() 
{
 money + upgrades;
 PlayerPrefs.SetInt("money", money);
}
public void upgrade() 
{
 while(money == 15)
 {
    upgrades++;
 }
PlayerPrefs.SetInt("upgrades", upgrades);
}

public void Save() 
{
 PlayerPrefs.SetInt("money", money);
 PlayerPrefs.SetInt("upgrades", upgrades);
}

}