title: How to make an arithmetic program that would print "invalid" if there's like double sign or invalid operation
body: <p>I'm trying to make an arithmetic program that would print <code>invalid</code> if there's like a double sign or invalid operation like (<code>++</code>, <code>--</code>, <code>+*</code>, etc) so I'm trying to write the if <code>elif</code> possibility one by one.</p>
  <p><a href="https://i.stack.imgur.com/WJwMt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WJwMt.png" alt="screenshot" /></a></p>
  <p>Here's the pict of the program that I just mentioned doing. So I was thinking if it's possible to write all of that in one <code>elif</code> and I try to write <code>elif</code> (<code>+-</code>, <code>-+</code>, <code>++</code>, <code>//</code>). I also tried <code>elif</code> (<code>+/</code> or <code>/+</code>) but none of them are works</p>