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I need this as a sentinel controlled loop. Any suggestions? am I on the right track?

<SCRIPT LANGUAGE = "JavaScript"> /* This code takes input from user, adds it all up and gives a total at the bottom

         * of all the products for seven days and the corresponding value of them all */





  var retail_price, validInput = true, counter = 1, total = 0, amount_sold = 0, total_retail_value = 0;



  do {

  pro_num = window.prompt ("Enter a product number (1-5)", "0");

  pro_num_value = parseInt(pro_num);

  

  

  

  

  switch (pro_num_value) {

   case "1":

    price = 2.98;

    break;

   case "2":

    price = 4.50;

    break;

   case "3":

    price = 9.98;

    break;

   case "4":

    price = 4.49;

    break;

   case "5":

    price = 6.87;

    break;

   default:

    validInput = false;

   }

  if ( validInput = true) {

  

    amount_sold = price * window.prompt("Enter number of products sold", "0");}

    total_retail_value += amount_sold;

  

  }

  while (pro_num != -1)

  



    document.writeln("Total Retail Value of all products sold over 7 days equals: " + total_retail_value);

  



  









</SCRIPT></code></p>