/* reset  */

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: arial;
}

body {
  text-align: center;
}

#calculator {
  width: 328px;
  height: auto;
  margin: 100px auto;
  padding: 20px;
  background-color: #444444;
 
}

/* Clear floats  */
.btns, .top {
    overflow: hidden;
}

.btn, .top span.clear {
  float: left;
  width: 66px;
  height: 40px;
  background: white;
  
  border-radius: 3px;
  margin: 0 8px 12px 0;
  line-height: 40px;
}

.btn-opr {margin-right: 0;}

/* top area */

.top span.clear {
  float: left;
}

.top .screen {
  height: 40px;
  width: 325px;
  background-color: rgba(0,0,0,0.2);
  line-height: 40px;
  color: white;
  text-align: right;
  padding-right: 50px;
}