Include sticky posts on index

Hello everyone! First of all sorry me for my bad English but isn’t my native language so I’ll do as best as possible to explain my problem.

I’ve a site running the last version of WordPress (4.6.1) and I want to display the sticky posts at the top of post loop on index.php. My theme’s code is the following:

<?php if (have_posts()) : ?>
<?php $post = $posts[0]; $c=0; ?>
<?php
if( is_home() && !is_paged() ) :
global $query_string;
query_posts( $query_string . '&cat=1,2&posts_per_page=7' );
endif;
?>
<?php /* Start the Loop */ ?>
<?php while (have_posts()) : the_post(); ?>
<?php $c++;
if( !$paged && $c == 1 ) :?>
FIRST POST STYLE

<?php else :?>

<?php get_template_part( 'content', get_post_format() ); ?>
<?php endif;?>
<?php endwhile; ?>

Is there any piece of code I’m missing? Thanks in advance! 🙂