<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="css/bootstrap.min.css">
<link rel="stylesheet" href="css/all.min.css">
<link rel="stylesheet" href="css/main.css">
</head>
<body>
<div class="container">
<div class="row">
<button class="btn btn-success start-btn" onclick="start()">Start</button>
</div>
<div class="row " id="cont">
<div id="div1" class="col-md-3">
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</div>
<div id="div2" class="col-md-3">
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</div>
<div id="div3" class="col-md-3">
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</div>
</div>
</div>
<script src="js/jquery-3.5.1.slim.min.js"></script>
<script src="js/popper.min.js"></script>
<script src="js/bootstrap.min.js"></script>
<script src="js/main.js"></script>
</body>
</html>