How can calculate automatically without click any button to run function

function daysBetween(startDate, endDate) {
  var millisecondsPerDay = 1000 * 60 * 60 * 24;
  var startDateUTC = Date.UTC(startDate.getFullYear(), startDate.getMonth(), startDate.getDate());
  var endDateUTC = Date.UTC(endDate.getFullYear(), endDate.getMonth(), endDate.getDate());

  return Math.floor((endDateUTC - startDateUTC) / millisecondsPerDay);
}

document.getElementById("btn").addEventListener("click", function() {
  let startDate = new Date(document.getElementById("startdate").value);
  let endDate = new Date(document.getElementById("enddate").value);
  let output = document.getElementById("totalprice");

  // Get how much days is between these dates
  let days = daysBetween(startDate, endDate);

  if (days) {
    output.classList.remove("text-danger");
    output.innerHTML = `${days*100}€`;
  } else {
    /* if no valid date is entered */
    output.classList.add("text-danger");
    output.innerHTML = `Select valid dates`;
  }

}, false);

<section class="sec5">Check in</section>
<section class="sec2"><input class="date" id="startdate" type="date" name="checkin" required=""></section>

<section class="sec5">Check out</section>
<section class="sec2"><input class="date" id="enddate" type="date" name="checkout" required=""></section>


<p>Total price
  <p>
    <section class="sec3" id="totalprice" name="totalprice">--</section>



    <div id="btn">CLICK ME!</div>