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

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

puts(sum/N)
