n = gets.to_i
x = []
y = []

n.times do
  line = gets.split.map(&:to_i)
  x << line.shift
  y << line.shift
end

#道の組み合わせ
per = []
(0..n-1).each do |i|
    per[i]=i
end
per=per.permutation(n).to_a
#p per

total=0
(0..per.size-1).each do |i|
    (0..per[i].size-2).each do |j|
        total+=((x[per[i][j]]-x[per[i][j+1]])**2+(y[per[i][j]]-y[per[i][j+1]])**2)**0.5
    end
end
#p total
p total/per.size
