def abc_145_c
    n = gets.chomp.to_i
    xy = []
    for i in 1..n
        xy.push(gets.chomp.split(/\s/).map{|x| x.to_f})
    end

    ave = 0.0
    brr = [*0..n-1].permutation(n).to_a
    brr.each do |arr|
        for i in 0..(arr.length-2)
            ave += Math.sqrt((xy[arr[i]][0] - xy[arr[i+1]][0])*(xy[arr[i]][0] - xy[arr[i+1]][0]) + 
                             (xy[arr[i]][1] - xy[arr[i+1]][1])*(xy[arr[i]][1] - xy[arr[i+1]][1]))
        end
    end

    puts ave/brr.length
end

abc_145_c