def factorial(number)
  if number.zero?
    1
  else
    number * factorial(number - 1)
  end
end

def foo(number)
  if number==2
    4
  else
    (number-1)*foo(number-1)
  end
end

N=gets.to_i
x=[]
y=[]
N.times do |i|
  x[i],y[i]=gets.chomp.split(" ").map{|j| j.to_i}
end

sum=0
num=0

N.times do |i|
  N.times do |j|
    next if i==j
    sum+=(Math.sqrt(((x[i]-x[j]).abs**2)+((y[i]-y[j]).abs**2)))*(foo(N)/4)
    num+=1
  end
end

puts "#{sum/factorial(N)}"
