n = gets.chomp.to_i
ans = 0
k = 1
a = Array.new(n).map{Array.new(2)}
for i in 0..n-1
  a[i][0],a[i][1] = gets.split.map(&:to_i)
end

for i in 1..n-1
  for j in 0..i-1
    d = (a[i][0]-a[j][0])**2+(a[i][1]-a[j][1])**2
    ans += Math.sqrt(d)
  end
end

puts ans*2/n
