FACT = []

def calc_fact(n)
  FACT[0] = 1
  for i in 1..n
    FACT[i] = FACT[i-1] * i
  end
end

n = gets.to_i
x = []
y = []
n.times do |i|
  x[i],y[i]=gets.chomp.split(" ").map(&:to_i)
end

a = []
n.times do |i|
  a.push(i+1)
end

a = a.permutation

sum = 0.0
a.each do |li|
  len = li.length
  for i in 0..len-2
    sum += ( ( x[li[i]-1]  - x[li[i+1]-1] ) ** 2.0 + ( y[li[i]-1] - y[li[i+1]-1]) ** 2.0 ) **(1/2.0)
  end
end

calc_fact(n)

puts sum / FACT[n]