height of box also changed

HTML

<div class=c1></div>
    <div class=c1></div>

CSS

.helper {
        border-style: dashed;
        border-width: thick;
      }

      .c1 {
        float: left;
        margin: 1em;
        width: 200px;
        height: 200px;
        background-color: red;
      }

JavaScript

$(
        $('.c1:last').resizable(
          {
            helper: 'helper'
          }
        )
      );