http://www.htmq.com/css3/border-radius.shtml
border-radius: 100px 25px 50px 50px / 50px 25px 50px 25px; を指定
<style>
.sample2 {
width:280px; height:130px; background-color:#ccecf4;
border:solid 20px #7fcfe2;
border-radius: 100px 25px 50px 50px / 50px 25px 50px 25px;
}
</style>
<p class="sample2">border-radius: 100px 25px 50px 50px / 50px 25px 50px 25px; を指定</p>