How make input radio value as number? [duplicate]

I am trying do arithmetic operation using radio button (just an experiment)

browser returns value NaN

let num1 = parseInt(document.querySelectorAll('#number1:checked'));
let num2 = parseInt(document.querySelectorAll('#number2:checked'));

function addTambah() {
  result = num1 + num2;
  alert(result);
}
<p>number : </p>
<input type="radio" name="number1" id="number11" value="1">
<label for="radio">1</label>
<input type="radio" name="number1" id="number12" value="2">
<label for="radio">2</label>
<p> number 2 : </p>
<input type="radio" name="number2" id="number21" value="1">
<label for="radio">1</label>
<input type="radio" name="number2" id="number22" value="2">
<label for="radio">2</label>
<br>
<button type="button" onclick="addTambah()">Add</button>