n = gets.to_i
cities = []
n.times { cities << gets.split.map(&:to_i) }
def distance(x1, y1, x2, y2)
  Math.sqrt((x1 - x2) ** 2 + (y1 - y2) ** 2)
end

array = []
cities.combination(2).each do |c|
  array << distance(c[0][0], c[0][1], c[1][0], c[1][1])
end

sum = array.inject(:+)
ans = sum / array.size * (n - 1)
puts ans
