n = gets.to_i

x = []
y = []

n.times { |i|
  items = gets.chomp.split(/ /).map { |e| e.to_i }
  x << items[0]
  y << items[1]
}

patterns = (1..n).to_a.permutation(n).to_a
length = 0

patterns.each { |pattern|
  (pattern.size - 1).times { |i|
    length += Math.sqrt((x[pattern[i]-1]-x[pattern[i+1]-1]) ** 2 + (y[pattern[i]-1]-y[pattern[i+1]-1]) ** 2)
  }
}

puts length / patterns.size

