How do I make a TextBox only accept numbers?

So I'm making a Light Placing plugin (I know, super original!) and when i click, I want a gui to appear asking you things like "How bright?". But, obviously, is someone typed letters in it, it would obviously error.

M39a9am3R is right.

--tonumber will turn a string into a number or return nil if the string is not a number
tonumber("hello!")--nil
tonumber("1244")-1244

Then to have the TextBox only use numbers, just do something like this.

textbox = script.Parent.TextBox --or something
textbox.FocusLost:connect(function()
	if tonumber(textbox.Text) then
		--do stuff with textbox.Text being a number
	end
end)

And if you wanted the players not be able to type letters, then do this.

textbox = script.Parent.TextBox --or something
textbox.Changed:connect(function(prop)
	if prop == "Text" then
		if not tonumber(textbox.Text) then
			textbox.Text = textbox.Text:sub(1,#textbox.Text-1)
			--using sub to delete the last thing in the string
			--since this fires every single time they type something, it should delete letters instantly
		end
	end
end)