# https://atcoder.jp/contests/abc145/tasks/abc145_c



N = gets.to_i
xs = Array.new(N)
ys = Array.new(N)
N.times do |i|
  xs[i], ys[i] = gets.split.map(&:to_i)
end

ans = 0
N.times do |i|
  i.times do |j|
    ans += Math.hypot(xs[i] -xs[j], ys[i] - ys[j])
  end
end

puts ans * 2 / N
