Not being able to complete this simple calculator exercise. I am getting Nan error - input

Question

I am writing a simple code from scratch and I get Nan error in console.log and undefined as a result.
Any suggestion to improve this code will be appreciated.
Many thanks.
<div>
<h1>Exercises</h1>
<h4>Items calculator</h4>
<p>Enter value</p>
<input type="number" id="numberitems">
<p>Enter price</p>
<input type="number" id="priceitems">
<br>
<br>
<button id="pricebtn">Calcular Precio</button>
<br>
<p>Resultado</p>
<span id="finalPrice"></span>
</div>
<script>
let items = document.getElementById("numberitems");
let price = document.getElementById("priceitems");
let finalPrice = items * price;
const btnprice = document.getElementById("pricebtn");
function myFunction1() {
document.getElementById("finalPrice").innerHTML += finalPrice.value;
}
btnprice.addEventListener("click", myFunction1);
console.log(finalPrice);
</script>

Answer 1

First of all: you are multiplying inputs, not its values.
To get values of input fields, you must get it with .value (not finalPrice.value)
As second: your price counted at the page loading, not on click event. The result is undefined, because items and price initializing at page startup, where they yet undefined.
To work it correct, you must getting the values and perform computations in function.
<script>
function myFunction1() {
let items = 0;
items = document.getElementById("numberitems").value;
let price = 0;
price = document.getElementById("priceitems").value;
let finalPrice = items * price;
document.getElementById("finalPrice").innerHTML += finalPrice;
console.log(finalPrice);
}
const btnprice = document.getElementById("pricebtn");
btnprice.addEventListener("click", myFunction1);
</script>

Answer 2

Try this:
let items = document.getElementById("numberitems").value;// ger value from this id
let price = document.getElementById("priceitems").value; // ger value from this id
let finalPrice = Number(items*price);
const btnprice = document.getElementById("pricebtn");
function myFunction1() {
document.getElementById("finalPrice").innerHTML += finalPrice;
}
btnprice.addEventListener("click", myFunction1);
console.log(finalPrice);