n=gets.to_i
x=Array.new(n,0.0)
y=Array.new(n,0.0)
n.times do |i|
    x[i],y[i]=gets.split.map(&:to_f)
end

ans=0
count=0
[*0..(n-1)].permutation.to_a.each do |perm|
    (n-1).times do |i|
        ans+=((x[perm[i+1]]-x[perm[i]])**2+(y[perm[i+1]]-y[perm[i]])**2)**0.5
    end
    count+=1
end

puts sprintf("%.7f", ans/count)
