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

body {
  height: 100vh;
  display: flex;
  align-items: center;
  background-color: #333;
  padding: 20px;
}

#calc-ctn {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  margin: auto;
  background-color: #000;
  border: 3px solid #fff;
  border-radius: 1rem;
}

#calc-screen {
  padding: 1rem;
}

#calc-screen .input,
#calc-screen .output {
  text-align: right;
  font-size: 4rem;
  border: none;
  width: 100%;
  padding: 0 1rem 2rem;
  border-radius: 0.2rem 0.2rem 0 0;
  background-color: #000;
  color: #fff;
}

#calc-screen .output {
  border-radius: 0 0 0.2rem 0.2rem;
}

.calc-btns,
.calc-advance {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  padding: 2rem;
}

.calc-advance {
  padding-left: 0;
}

#footer {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
}

.group-btn {
  display: flex;
  gap: 0.8rem;
}

.btn {
  font-size: 2rem;
  text-align: center;
  border: none;
  border-radius: 50%;
  width: 4rem;
  height: 4rem;
  color: #fff;
}

.btn:hover {
  transform: translate(0, -2px);
  transition: 0.5s;
}

.btn:active {
  transform: translate(0, 2px);
}

.btn-grey {
  background-color: #808080;
}

.btn-orange {
  background-color: #ffa500;
}

.btn-dark-grey {
  background-color: #303030;
}

.backspace {
  flex-grow: 1;
  border-radius: 2rem;
  display: flex;
  align-items: center;
  justify-content: center;
}

.hidden {
  display: none;
}