Place ads between two text-only paragraphs - php

Question

I am using the following code to place ads between text-only paragraphs. However, the problem is it inserts only 2 ads in the content even if the post is lengthy.
So, I want to display the ad in every nth paragraph (keeping the first ad). So, if n=4, the ad will display after the 4th paragraph but only if the 5th paragraph have text.
add_filter( 'the_content', 'so_25888630_ad_between_paragraphs' );
function so_25888630_ad_between_paragraphs($content){
if( in_the_loop() ){
$closing_p = '</p>';
$paragraphs = explode( $closing_p, wptexturize($content) );
$count = count( $paragraphs );
if( 4 >= $count ) {
$totals = array( $paragraphs );
}else{
$midpoint = floor($count / 2);
$first = array_slice($paragraphs, 0, $midpoint );
if( $count%2 == 1 ) {
$second = array_slice( $paragraphs, $midpoint, $midpoint, true );
}else{
$second = array_slice( $paragraphs, $midpoint, $midpoint-1, true );
}
$totals = array( $first, $second );
}
$new_paras = array();
foreach ( $totals as $key_total=>$total ) {
$p = array();
foreach ( $total as $key_paras=>$paragraph ) {
$word_count = count(explode(' ', $paragraph));
if( preg_match( '~<(?:img|ul|li)[ >]~', $paragraph ) || $word_count < 10 ) {
$p[$key_paras] = 0;
}else{
$p[$key_paras] = 1;
}
}
$m = array();
foreach ( $p as $key=>$value ) {
if( 1 === $value && array_key_exists( $key-1, $p ) && $p[$key] === $p[$key-1] && !$m){
$m[] = $key;
}elseif( !array_key_exists( $key+1, $p ) && !$m ) {
$m[] = 'no-ad';
}
}
if( $key_total == 0 ){
$ad = array( 'ad1' => '<p>PLACE YOUR ADD NO 1 HERE</p>' );
}else{
$ad = array( 'ad2' => '<p>PLACE YOUR ADD NO 2 HERE</p>' );
}
foreach ( $total as $key_para=>$para ) {
if( !in_array( 'no_ad', $m ) && $key_para === $m[0] ){
$new_paras[key($ad)] = $ad[key($ad)];
$new_paras[$key_para] = $para;
}else{
$new_paras[$key_para] = $para;
}
}
}
$content = implode( ' ', $new_paras );
}
return $content;
}