n = gets.to_i
n_fact = (1..n).inject(1, &:*)
mul = n_fact / n
cities = (1..n).map { gets.split.map(&:to_i) }
perm = cities.permutation(2)
total = perm.map do |seq|
  a, b = seq 
  (Math.sqrt((a[0] - b[0])**2 + (a[1] - b[1])**2))*mul
end.inject(&:+)

puts total / n_fact