add event listener to php form

Hi experts, I am using bootstrap 3
I have a page with 4 dropdown options and 4 number INPUT with different ids (see dropdown 1 below)
How can I add an eventlistener to either the dropdown or number input changes its value ?
What I am trying to accomplish is calculate the price on the fly (ajax and php)
P.S. I can calculate the price IF THE USER CLICKS A BUTTON, but I want to do the same calculations WITHOUT THE USER CLICK THE CALCULATE PRICE BUTTON

Tnx for any help or suggestions


    <div class="form-row">
     <div class="form-group col-sm-5">
        <label for="sel1" class="control-label">Select list :</label>
        <select class="input-large" id="sel1">
          <option value="399">Nativity Set - 399.00&#160;&#160; </option>
          <option value="189">Deer Set - 189.00&#160;&#160; </option>
          <option value="35">Sheep - 35.00&#160;&#160;</option>
          <option value="35">Cow - 35.00&#160;&#160;</option>
          <option value="35">Donkey - 35.00&#160;&#160;</option>
          <option value="35">Goat - 35.00&#160;&#160;</option>
        </select>
     </div>
       
     <div class="form-group">
      <label for="iQ1" class="col-sm-2 control-label">Quantity</label>
      <div class="col-sm-2">
        <input type="number" class="form-control" id="iQ1" value="1">    
      </div>  
     </div>
    </div>