Why I have not a number (NaN) output and not correct number

function sum() {
  var num1 = parseFloat(document.getElementById('bene_significativo').value);
  var num2 = parseFloat(document.getElementById('manodopera').value);
  var num3 = parseFloat(document.getElementById('mark_up').value);

  document.getElementById('tot1').value = num1;
  document.getElementById('tot2').value = num2 + num3;
  document.getElementById('tot3').value = num1 + num2 + num3;

  var iva10 = document.getElementById('iva10').value = num1 - tot2;
}

Bene significativo: <input type="text" id="bene_significativo" class="form-control" placeholder="Valore del bene significativo" aria-label="bene significativo" /><br />
Manodopera: <input type="text" id="manodopera" class="form-control" placeholder="Default input" aria-label="mandopera" /><br />
Mark-up: <input type="text" id="mark_up" class="form-control" placeholder="Default input" aria-label="mark_up" /><br />
<br />
<div><button class="btn btn-primary" onclick="sum()">Calcola</button></div>
<br />
Bene significativo: <input type="text" value="" id="tot1" /><br />
Manodopera: <input type="text" value="" id="tot2" /><br />
Totale complessivo prestazione: <input type="text" value="" id="tot3" /><br />
IVA 10%: <input type="text" value="" id="iva10" /><br />
IVA 22%: <input type="text" value="" id="iva22" />