n = gets.to_i
point = []
n.times {point << gets.split.map(&:to_i)}
fac = [1,1,2,6,24,120,720,5040,40320]

sum = 0.0

(0..n-1).to_a.permutation do |arr|
    (n-1).times do |i|
        sum += Math.sqrt((point[arr[i+1]][0] - point[arr[i]][0]) ** 2 + 
                         (point[arr[i+1]][1] - point[arr[i]][1]) ** 2)
    end
end

puts sum / fac[n]