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]-b[0])**2 + (a[1]-b[1])**2)**(1/2.0))
    end
    path_len.push(lens.inject(:+))
end
print("#{path_len.inject(:+) / path_len.length}")