@function area-do-circulo($raio)
  $pi: 3.14159
  @return $pi * $raio * $raio

.borda 
  border: area-do-circulo(5) * 1px solid #3498db
  padding: 20px
  margin: 20px
  text-align: center
  background-color: #f0f0f0
  border-radius: 10px
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