n = gets.chomp.to_i
data = n.times.map do
    gets.chomp.split(" ").map(&:to_i)
end

pat = 1

for i in 1..n do
    pat = pat * i
end

distancelist = Array.new()

data.permutation(n) do |arr|
    red = 0.0
    dist = 0.0
    sq = 0
    for j in 1..n-1 do
        x = arr[j - 1][0]
        y = arr[j - 1][1]
        xn = arr[j][0]
        yn = arr[j][1]
        sq = (x - xn)*(x - xn) + (y - yn)*(y - yn)
        red += Math.sqrt(sq)
    end
    distancelist.push(red)
end

result = (distancelist.sum / pat)
puts result