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

if (reloading)
{
if (currentAmmo <= 0)
{
if (ammoInventory >= 40)
{
currentAmmo += 40;
ammoInventory -= 40;
} else
{
currentAmmo += ammoInventory;
ammoInventory = 0;
}
}
}