p {
  margin: 0;
}
html {
  box-sizing: border-box;
  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;;
  color: #3E3E3D;
}
*, *:before, *:after {
  box-sizing: inherit;
}


.calculator {
  margin: 10px auto;
  height: 500px;
  width: 400px;
  background-color: #DBB92E;
}

/* screen */
.screen {
  height: 100px;
  background-color: #eee;
  overflow: auto;
}
.screen p {
  font-size: 40px;
  text-align: right;
  line-height: 100px;
  padding: 0 20px;
}


.left-panel {
  float: left;
  font-size: 40px;
  width: 300px;
}
.key {
  position: relative;
  float: left;
  transition: background-color 0.3s ease;
}
.key:hover {
  background-color: rgba(0, 0, 0, 0.1);
}

.left-panel .key {
  width: 100px;
  height: 100px;
}
.right-panel .key {
  width: 100px;
  height: 66.666667px;
}
.key p {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}
.right-panel {
  float: right;
  font-size: 25px;
  width: 100px;
}

footer {
  text-align: center;
  margin-top: 50px;
}