n = gets.to_i
towns = (0..n).to_a
array = []
n.times do
x,y = gets.split(" ").map {|v| v.to_i}
array << [x,y]
end
d = 0
c_towns = towns.clone
while c_towns.size >= 2
  a = c_towns.pop
  d1 = 0
  for b in c_towns
    dis = (towns[a][0] - towns[b][0]) ** 2 + (towns[a][1] - towns[b][1]) ** 2
    d1 += Math.sqrt(dis)
  end
  i = 1
  (1...(c_towns.size - 1)).to_a.each {|v| i *= v}
  d += (d1 *  i)
end
i = 1
((1...n).to_a.each {|v| i *= v})
puts d / i