*, *::before, *::after {
  box-sizing: border-box;
}

body {
  background: #000;
  font-family: 'Microsoft YaHei','Lantinghei SC','Open Sans',Arial,'Hiragino Sans GB','STHeiti','WenQuanYi Micro Hei','SimSun',sans-serif;
  font-size: 16px;
}

.calculator {
  width: 30rem;
  max-width: 100%;
  margin: 3rem auto;
  padding: .3rem;
}
.calculator .display {
  height: 4rem;
  line-height: 4rem;
  margin: 0 .3rem;
  padding: 0 0;
  text-align: right;
  font-size: 1.5em;
  color: #fff;
  border-bottom: 1px solid rgba(255, 255, 255, 0.15);
}
.calculator .brand {
  margin: .6rem 0 0.3rem;
  padding: 0 .3rem;
  text-align: right;
  font-size: .65em;
  color: rgba(255, 255, 255, 0.85);
}
.calculator .keyboard {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-wrap: wrap;
      flex-wrap: wrap;
}
.calculator .keyboard .btn {
  margin: 0;
  padding: 0;
  height: 4.6rem;
  line-height: 4rem;
  text-align: center;
  font-family: inherit;
  font-size: 1.25em;
  border: none;
  background: transparent;
  outline: none;
}
.calculator .keyboard .btn:hover span {
  background: #4d4d4d;
}
.calculator .keyboard .btn span {
  display: block;
  margin: .3rem;
  color: #fff;
  background: #333;
  border-radius: 3px;
  -webkit-transition: background-color .4s;
  transition: background-color .4s;
}
.calculator .keyboard .btn.primary:hover span {
  background: #6bb6ff;
}
.calculator .keyboard .btn.primary span {
  background: dodgerblue;
}
.calculator .keyboard .panel-a {
  width: 75%;
}
.calculator .keyboard .panel-a .top-buttons .btn {
  width: 33.3%;
}
.calculator .keyboard .panel-a .top-buttons .btn:last-of-type {
  float: right;
}
.calculator .keyboard .panel-a .top-buttons .btn span {
  font-weight: bold;
}
.calculator .keyboard .panel-a .numbers {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-wrap: wrap;
      flex-wrap: wrap;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: reverse;
      -ms-flex-direction: row-reverse;
          flex-direction: row-reverse;
  -webkit-box-align: end;
      -ms-flex-align: end;
          align-items: flex-end;
  -ms-flex-line-pack: end;
      align-content: flex-end;
}
.calculator .keyboard .panel-a .numbers .btn {
  width: 33.3%;
}
.calculator .keyboard .panel-a .numbers .btn:nth-child(10) {
  width: 100%;
}
.calculator .keyboard .panel-b {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
      -ms-flex-direction: column;
          flex-direction: column;
  width: 25%;
}
.calculator .keyboard .panel-b .btn {
  width: 100%;
}
.calculator .keyboard .panel-b .btn span {
  font-weight: bold;
}