include Math
N = gets.to_i
x = []
y = []
N.times do |i|
  x[i], y[i] = gets.split.map(&:to_i)
end

def dist(x1, y1, x2, y2)
  Math.sqrt((x1-x2)**2 + (y1-y2)**2)
end

# 1からNまでのN!この順列を生成
p = []
1.upto N do |i|
  p[i-1] = i
end
p = p.permutation(N).to_a

ans = 0
p.each do |r|
  (N-1).times do |i|
    ans += dist(x[r[i]-1], y[r[i]-1], x[r[i+1]-1], y[r[i+1]-1])
  end
end
# (1..N).inject(1,:*) は N の階乗
puts ans.to_f / p.size.to_f