text: ﻿function inputLine(form) {
      var firstNumber = form.firstNumber.value*1;
      var secondNumber = form.secondNumber.value*1;
      var thirdNumber = form.thirdNumber.value*1;
      if (!isNaN(firstNumber) && firstNumber !== ''
          && !isNaN(secondNumber) && secondNumber !== ''
         && !isNaN(thirdNumber) && thirdNumber !== '') {
          if (firstNumber > 0 && secondNumber > 0 && thirdNumber > 0) {
              alert("The sign of the product is +")
          }
          else if (firstNumber < 0 && secondNumber > 0 && thirdNumber > 0) {
              alert("The sign of the product is -")
          }
          else if (firstNumber > 0 && secondNumber < 0 && thirdNumber > 0) {
              alert("The sign of the product is -")
          }
          else if (firstNumber > 0 && secondNumber > 0 && thirdNumber < 0) {
              alert("The sign of the product is -")
          }
          else if (firstNumber < 0 && secondNumber < 0 && thirdNumber > 0) {
              alert("The sign of the product is +")
          }
          else if (firstNumber < 0 && secondNumber > 0 && thirdNumber < 0) {
              alert("The sign of the product is +")
          }
          else if (firstNumber > 0 && secondNumber < 0 && thirdNumber < 0) {
              alert("The sign of the product is +")
          }
          else if (firstNumber < 0 && secondNumber < 0 && thirdNumber < 0) {
              alert("The sign of the product is -")
          }
          else if (firstNumber === 0 || secondNumber === 0 || thirdNumber === 0) {
              alert("The product is 0")
          }
      }
      else {
          alert("Not valid numbers.")
      }
  }