Ammo Count Trouble....

I have an Ammo Counter established on my PlayGui. I named it AmmoCounter. The problem that I am having is how to write the functions to register the initial start amount of 30 and every time the weapon is fired the count decreases and every time the player picks up an ammo refill it increases the counter by 20. The player initially spawns into the game with 30 rounds of m16 ammo but has a max inventory allowed of 210. Thanks in advance.