How does on go about doing an mathematical operation on a string value?

I'm trying to get this script to put a decal on a screen, from a gui textbox, but the output tells me that i cannot do math on a string value. Any help is most appreciated

button = script.Parent
DecalId = script.Parent.DecalId
screen1 = game.Workspace.screen1.Decal.Texture
screen2 = game.Workspace.screen2.Decal.Texture

function onclicked()
	screen1 = "http://www.roblox.com/asset/?id="..button.idInput.Text - 1
	screen2 = "http://www.roblox.com/asset/?id="..button.idInput.Text - 1
end

script.Parent.idInput.MouseButton1Down:connect(onclicked)

You have to convert a string to a number to do math on it, use tonumber() :

ExampleString = "300"
print(tonumber(ExampleString) - 100) --Would print 200

So in your case:

screen1 = "http://www.roblox.com/asset/?id="..(tonumber(button.idInput.Text) - 1)
screen2 = "http://www.roblox.com/asset/?id="..(tonumber(button.idInput.Text) - 1)

You're concatenating it before you're subtracting one. Put the number in parenthesis with the math. Lua automatically converts numerical strings into numbers when you do math on them.