N,*xys=$<.read.split.map(&:to_i)
xys=xys.each_slice(2).to_a

sum = 0
count = 0
(0..N-1).to_a.permutation do |is|
    dist=0
    is.each_cons(2) do |ab,cd|
        a,b=xys[ab]
        c,d=xys[cd]
        dist += Math.sqrt((a-c)**2+(b-d)**2)
    end
    sum += dist
    count += 1
end

puts(sum/count)
