n = gets.to_i

pos = []
n.times do |i|
  pos << gets.split.map(&:to_i)
end

ans = 0.0
cnt = 0
a = []
n.times do |i|
  a << i
end
aa = a.permutation.to_a
#p aa
aa.each do |t|

  cur = pos[t[0]]
  tans = 0.0
  1.upto(n-1) do |j|
    nxt = pos[t[j]]
    tans += ((cur[0] - nxt[0])**2 + (cur[1] - nxt[1])**2)**0.5
    cur = nxt
  end
  ans += tans
  cnt += 1
end
p ans/cnt