n = gets.to_i
n.times do
x,y = gets.split(" ").map {|v| v.to_i}
array << [x,y]
end
dis = 0
for i in 0...(array.size - 1)
d = (array[i+1][0] - array[i][0]) ** 2 + (array[i+1][1] - array[i][1]) ** 2
dis += Math.sqrt(d.to_f)
end
puts dis / (n - 1)