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All You have to edit your WordPress Theme’s index.php file
1- Login to WordPress Admin Panel and navigate to Appearance > Theme Editor
2- Now, Find and click on index.php (Main Index Template ) template from right listed files list.
3- Find This Code
<?php if (have_posts()) : ?>
4- Above given code with following code
<?php if (have_posts()) : ?>
<?php $count = 1; ?>
5- Now find following code
<div class=”entry”>
or
<div class=”postcontent”>
6- Add following code after above searched code.
<?php if ($count == 1) : ?>
AdSense Code
<?php endif; $count++; ?>
By defafult the add will be displayed after 1st post, BUT If you want the add under desired number of post, simple change $count == 2 the figure.