N = gets.to_i
XY = $<.map{|s|
  x, y = s.split.map &:to_i
}
sum = 0
cnt = 0
XY.permutation(N){|perm|
  perm.each_cons(2){|(x1, y1), (x2, y2)|
    sum += ((x2 - x1) ** 2 + (y2 - y1) ** 2) ** 0.5
  }
  cnt += 1
}
p sum / cnt
