def length(a, b)
  ((a[0] - b[0])**2 + (a[1] - b[1])**2)**0.5
end

N = gets.chomp.to_i
pos = []
N.times { pos << gets.chomp.split(" ").map(&:to_i) }

a = []
[*(0...N)].permutation(2) do |b, c|
  a << length(pos[b], pos[c])
end
mean = a.inject(0, :+) / a.size

puts mean * (N - 1) 