n = gets.chomp.to_i
points = readlines.map{ |line| line.chomp.split.map(&:to_i) }
 
permutation = [*0..n-1].permutation(n).to_a

sum = 0

permutation.each do |root|
    distance = 0
    root.each_with_index do |point, index|
        if index != 0
            distance += Math.sqrt((points[point][0] - points[root[index-1]][0])**2 + (points[point][1] - points[root[index-1]][1])**2)
        end
    end
    sum += distance
end

puts sum / permutation.size