how do i make a gui only work when theres something in the input

im trying to make an exchange block where i put in 10 of the item and i get 1 emerald for it but i cant figure out how to make the gui only give me the emerald when there is the 10 items in the input slot. i understand how to make the gui take the 10 items and give me an emerald but it doesnt recognize when 10 items are in the slot or not and gives me an emerald when i press the button regardless of how much is in the slot