the codes listed here is part of the following article : https://www.mwordpress.net/how-to-add-adsense-into-post-wordpress/

content-filter.php

<?php
$content 	= apply_filters('the_content', $post->post_content);
$save		= explode("</p>", $content);
$tcount		= 0;
$adon		= 0;
foreach($save as $item) {
    echo $item;
    echo "</p>";
    if(preg_match('/<p> /',$item) == 0 && $tcount >= 1 && $adon == 0) {
		$adon=1;
		?><div class="ad-code">هنا يمكن وضع اعلانات جوجل ادسنس</div><?php
    }
	if(preg_match('/<p> /',$item)==0 && $tcount>=4 && $adon==1) {
		$adon=2;
		?><div class="ad-code">هنا يمكن وضع اعلانات جوجل ادسنس</div><?php
	}
	$tcount++;
}
?>

insert-ads

//Insert ads after second paragraph of single post content.
add_filter( 'the_content', 'prefix_insert_post_ads' );
function prefix_insert_post_ads( $content ) {

    $paragraph = get_post_meta($post->ID, 'mw_paragraph' , true); // by mwordpress.net
    $hide = get_post_meta($post->ID, 'mw_hide_ads' , true); // by mwordpress.net

    if (!$hide) :
    $ad_code = ads_code_print();
    endif;

    if ( is_single() && ! is_admin() ) {
        if (!$paragraph) {
            return prefix_insert_after_paragraph( $ad_code, 2, $content );
        } else {
            return prefix_insert_after_paragraph( $ad_code, $paragraph, $content );
        }
    }

    return $content;
}
// Parent Function that makes the magic happen
function prefix_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
    $closing_p = '</p>';
    $paragraphs = explode( $closing_p, $content );
    foreach ($paragraphs as $index => $paragraph) {

        if ( trim( $paragraph ) ) {
            $paragraphs[$index] .= $closing_p;
        }

        if ( $paragraph_id == $index + 1 ) {
            $paragraphs[$index] .= $insertion;
        }
    }
    return implode( '', $paragraphs );
}
function print_ads_code() {
echo '<script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<ins class="adsbygoogle"
     style="display:block"
     data-ad-client="ca-pub-XXXXXXXXXXXXXXX"
     data-ad-slot="XXXXXXXXXX"
     data-ad-format="auto"></ins>
<script>
(adsbygoogle = window.adsbygoogle || []).push({});
</script>';
}

the_content.php

<?php the_content(); ?>