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There are another way how to insert banner or text script advertisement after first paragraph insingle post content of WordPress blog. Just copy this code below:

<?php
    $paragraphAfter= 1;
    $content = apply_filters('the_content', get_the_content());
    $content = explode("</p>", $content);
    for ($i = 0; $i <count($content); $i++) {
    if ($i == $paragraphAfter) { ?>
<div>INSERT YOUR ADS SCRIPT HERE</div>
    <?php
    }
    echo $content[$i] . "</p>";
    } ?>
									

And replace the code

<?php the_content ?>
									

in single.php file of your WordPress theme.

Source code
http://wordpress.org/support/topic/determine-automatically-the-middle-of-the-post-from-word-count