how to increase or decrease ammo ?? but depending of ammo in inventory

so i want to make ammo script but i confused when increase or decrease ammo,

 int currentAmmo; //i already make maksimum is 40 and minimum is 0
 int ammoInventory; //and i make maksimum is 500 and minimum is 0
 
 //so i want to add currentAmmo, but depending of ammoInventory
 
 if (reloading)
 {
         if (currentAmmo <= 0 && ammoInventory > 40)
         {
                 //i make simple increase and decrease
                 currentAmmo = 40;
                 ammoOnInventory -= 40;
                 //how can i make if currenAmmo = 15 and ammoInventory = 10
                 //currentAmmo should only be added 10
         }
 }

thanks