如何使我的下拉选择按它们的值更改合计，但除非选中，否则不会计算

<select id="mySelect" onchange="calculateTotal()">
  <option value="0">请选择</option>
  <option value="10">10</option>
  <option value="20">20</option>
  <option value="30">30</option>
</select>

<span id="total">合计：0</span>

function calculateTotal() {
  var select = document.getElementById("mySelect");
  var totalElement = document.getElementById("total");
  
  var selectedValue = parseInt(select.value);
  var total = 0;
  
  if (selectedValue !== 0) {
    total = selectedValue;
  }
  
  totalElement.textContent = "合计：" + total;
}

<select id="mySelect" onchange="calculateTotal()">
  ...
</select>