Challenge #1 of 2

Q: 

The dog-walking service is a hit! So, we've added Bear to the list:

<?php
$walker1 = 'Kitty';
$walker2 = 'Tiger';
$walker3 = 'Jay';

$dogWalkers = [$walker1, $walker2, $walker3];
$dogWalkers[] = 'Bear';
?>

What array key will PHP automatically assign to Bear?