<link href="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<div class="container">
<header class="text-center">
<h1>Mesonry Image gallery</h1>
</header>
<div class="row">
<div class="col-md-8 col-sm-12 co-xs-12 gal-item">
<div class="row h-50">
<div class="col-md-12 col-sm-12 co-xs-12 gal-item">
<div class="box">
<img src="http://fakeimg.pl/758x370/" class="img-ht img-fluid rounded">
</div>
</div>
</div>
<div class="row h-50">
<div class="col-md-6 col-sm-6 co-xs-12 gal-item">
<div class="box">
<img src="http://fakeimg.pl/748x177/" class="img-ht img-fluid rounded">
</div>
</div>
<div class="col-md-6 col-sm-6 co-xs-12 gal-item">
<div class="box">
<img src="http://fakeimg.pl/371x370/" class="img-ht img-fluid rounded">
</div>
</div>
</div>
</div>
<div class="col-md-4 col-sm-6 co-xs-12 gal-item">
<div class="col-md-12 col-sm-6 co-xs-12 gal-item h-25">
<div class="box">