N = gets.to_i
info = readlines

num = 1
arr = []
ans = []
sum = 0

N.times do
    arr.push(num)
    num += 1
end

paterns = arr.permutation(N).to_a
paterns.each do |patern|
    index = 0
    (N-1).times do
        x = info[patern[index]-1].split(" ")[0].to_i - info[patern[index+1]-1].split(" ")[0].to_i
        y = info[patern[index]-1].split(" ")[1].to_i - info[patern[index+1]-1].split(" ")[1].to_i
        sum += Math.sqrt(x*x + y*y)
        index += 1
    end
end

puts sum/paterns.count  

