n = gets.to_i
ary = []

n.times do |i|
  xy = gets.chomp.split(" ").map(&:to_i)
  ary.push(xy)
end

#p ary,ary[2][1]

cmb = n*(n-1)/2
#p Math.sqrt(2) 
sum = 0

(n-1).times do |i|
  (n-i-1).times do |j|
    #p [i,j+i+1]
    x2 = (ary[i][0] - ary[j+i+1][0])**2
    y2 = (ary[i][1] - ary[j+i+1][1])**2
    xy2 = Math.sqrt(x2 + y2)
    #p [x2, y2, xy2]
    sum += xy2
  end
end

puts sum*(n-1)/cmb