Alert messages; Help?

Hi there,

I am trying to make alert messages for when neither a flight nor seating preference is made, and I have no idea how. Any help would be amazing. Also, if there is anything that can be done to improve this, please mention.

<!DOCTYPE html>
<meta http-equiv="Content-Type" content="text/html;
<title>gotham airlines</title>

function calculateCost() {

for (var i = 1; i < 7; i++) {
var radiobutton = document.getElementById("flight" + i);
if (radiobutton.checked == true) {
var base=radiobutton.value;
var rtn=document.getElementById("returnfare").checked?2:1;
for (var a = 1; a < 4; a++) {
var radiobutton = document.getElementById("seating" + a);
if (radiobutton.checked == true) {
var seatclass=radiobutton.value;
var cost=base*rtn*seatclass;
alert("The selected flight will cost $" + cost);

<h1>Gotham Airlines Fare Calculator</h1>
  <p>Complete the form below to calculate the cost of your flight.</p>
  <p><input type="radio" id="flight1" name="flights"
value="230"> <label for="flight1">Gotham - Hill Valley</label><br>
  <input type="radio" id="flight2" name="flights"
value="250"> <label for="flight2">Gotham - Las Venturas</label><br>
  <input type="radio" id="flight3" name="flights"
value="190"> <label for="flight3">Gotham - Storybrooke</label><br>
  <input type="radio" id="flight4" name="flights"
value="160"> <label for="flight4">Gotham - Marlinshire</label><br>
  <input type="radio" id="flight5" name="flights"
value="270"> <label for="flight5">Gotham - Lillian</label><br>
<input type="radio" id="flight6" name="flights"
value="220"> <label for="flight6">Gotham - Seahaven</label></p>

  <p><input type="checkbox" id="returnfare" name="return fare"
value="2"> <label for="returnfare">Click here if you will be purchasing a return fare</label></p>

  <p><input type="radio" id="seating1" name="seating"
value="2"> <label for="seating1">First class</label><br>
  <input type="radio" id="seating2" name="seating"
value="1.5"> <label for="seating2">Business class</label><br>
  <input type="radio" id="seating3" name="seating"
value="1"> <label for="seating3">Economy class</label></p>
  <p><input type="submit" value="Calculate"
onClick="calculateCost();"> <input type="reset"></p>

The first thing to do would be to replace the debugging alert statements with code that inserts the error message into the actual page itself. The alert() box displays a checkbox in some browsers allowing JavaScript to be turned off for the page or to simply skip over any subsequent alert boxes. Its ise for anything other than debugging disappeared about the same time that Netscape 4 did.

I am quite new to javascript, and I feel slightly silly that I have no idea what you said there.