Jquery does not work with the input loaded with Ajax

The jQuery function works normally but by including input ajax jQuery does not work for the input that has been included!

I have the function:

$('.alerta').on('input', function() { 
    alert('Olá mundo!'); 
})

I have input 1 working with the function above:

<div>
    Número 1:<br>
    <input class="alerta" type="text" name="x"/><br><br>
</div>
<div class="div">
</div>

I have ajax that includes input number 2 in classe="div" , but that does not work with the jQuery function.

<?php
echo 'Número 2:<br>
<input class="alerta" type="text" name="y"/><br><br>';
?>