try to get people input 2 numbers and add the sum but code doesnt work pls help thanks heaps

Question

my code below doesnt work just to add sum for Num1 and Num2.

Num 1

<label>Num 2 </label><input type="text" id="num2">  

<h1>Result : <span id="demo"> </span></h1>  

<script>
  let Num1= document.getElementById("num1").value;
  let Num2= document.getElementById("num2").value;
 
  function Sum(Num1,Num2){return (Num1+Num2)}

  let a=Sum(Num1,Num2)

  document.getElementById("demo").innerText=a

</script>