# coding: utf-8
N = gets.to_i
X = []
Y = []
N.times do |_|
  _x, _y = gets.split.map {|e| e.to_i}
  X << _x
  Y << _y 
end
ansA = []

# 順列分繰り返し
towns = (0..N-1).to_a
towns.permutation(towns.size).each do |ptn|
  _m = ptn.size - 2
  _sum = 0.0
  (0.._m).each do |i|
    fi = ptn[i]
    ti = ptn[i+1]
    _sum += Math.sqrt(((X[fi] - X[ti])**2) + ((Y[fi] - Y[ti])**2))
  end
  ansA << _sum
end

# 平均値を算出
puts ansA.inject(:+) / ansA.size

