gistfile1.php

<?php

add_filter( 'the_content', 'sp_adsense_first_paragraph' );
function sp_adsense_first_paragraph ( $content ) {
        if ( !is_single() ) 
            return $content;

            $paragraphAfter = 2; //Enter paragraph number
            $content = explode ( "</p>", $content );
            $new_content = '';
                for ( $i = 0; $i < count ( $content ); $i ++ ) {
                    if ( $i == $paragraphAfter ) {
                    $new_content .= '<div style="width: 600px; height: 100px; padding: 5px 5px 5px 0; margin-right: 0;">';
                    $new_content .= 'Paste Adsense Code Here';
                    $new_content .= '</div>';
                    }
            $new_content .= $content[$i] . "</p>";
            }
            return $new_content;
}