I'm learning Javascript right now and I got a problem (might be a small problem for you guys). Here is my code:`

```
<!DOCTYPE html>
<html>
<body>

<input type="number" id="number1" placeholder="like 0,25" />
<input type="number" id="number2" placeholder="like 200" />
<input type="number" id="number3" placeholder="like 0,30"/>

<button onclick="myFunction()">Try it</button>

<p style="padding: 10px;
        width: 2em;
          border:3px green solid;" id="demo"></p>
<script>

function myFunction() {
    var x = document.getElementById("number1").value;
    var y = document.getElementById("number2").value;
    var z = document.getElementById("number3").value;

    var ergebnis = Math.round(eval("(x*x*y)/(z*z)"));
    var res = ergebnis;
    document.getElementById("demo").innerHTML = res;
    } 

</script>

</body>
</html>

```

Now I want that "error" will be displayed when the button is clicked and there is an empty input field. And that the `<p>` will be first displayed, when the button is clicked. I know that I can hide the `<p>` but I don't know how to make it visible after the click again.