n = gets.to_i
a = []
sum = 0.0
k = 1
(n-1).times do |i|
    k*=(i+1)
end
n.times {a.push(gets.split.map(&:to_i))}
n.times do |i|
    (i+1).upto(n-1) do |j|
        x = ((a[i][0]-a[j][0])**2).to_f
        y= ((a[i][1]-a[j][1])**2).to_f
        sum += 2*k*(x+y)**(1/2.0)
    end
end

p sum/k/n