N = gets.to_i
cities = []
N.times do |c|
    cities.push(gets.chomp.split(" ").map(&:to_i))
end

path_len = []
perm = cities.permutation(N).to_a
for item in perm
    lens = []
    item.each_cons(2) do |a, b|
        lens.push(((a[0]-a[1])**2 + (b[0]-b[1])**2)**(1/2.0))
    end   
    path_len.push(lens.inject(:+))
end
print("#{path_len.inject(:+) / path_len.length}")