def factorial(n)
  i = 1
  k = 1
  while i < n
    k *= i+1
    i += 1
  end
  k
end
n = gets.to_i
arr = []
n.times do
  arr.push(gets.chomp.split(' ').map(&:to_i))
end
i = 0
sum = 0
while i < n - 1
  j = i + 1
  while j < n
    sum += Math.sqrt((arr[i][0] - arr[j][0])**2 + (arr[i][1] - arr[j][1])**2)
    j += 1
  end
  i += 1
end
puts (sum * 2)/n
