Why is my JavaScript is returning NaN when I run the code?

I started learning programming a little while ago, but I got stuck on the functions and intervals part.

What I wanted was a very simple system, to do multiplication, using HTML and JavaScript, passing the input numbers as parameters.

However, when I run the code, it returns NaN as the result.

let button = document.querySelector("#button");
let num1 = parseInt(document.getElementById('num1').value);
let num2 = parseInt(document.getElementById('num2').value);

function operacao(a, b) {
  let resp = a * b;
  return resp;
}

let resto = operacao(num1, num2)
button.addEventListener("click", function() {
  document.getElementById('resto').innerHTML = 'O resultado é: ' + resto
});

<label>
  Informe os numeros:  
  <br>
  <input type="number" id="num1">
  <p>x</p>
  <input type="number" id="num2">
</label>
<br>
<br>
<button id="button" onclick=(operacao)>Multiplicar</button>
<p id="resto"></p>