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)