n = gets.chomp.to_i
xx = []
yy = []
n.times do |i|
    x,y = gets.chomp.split(" ").map(&:to_i)
    xx << x
    yy << y
end

arr = [*1..n]
ans = 0
count = 0
arr.permutation do |ar|
    count += 1
ar.each_cons (2) do |i,j|
    ans += ((xx[j-1] - xx[i-1]) ** 2 + (yy[j-1] - yy[i-1]) ** 2) ** 0.5
end
end
puts ans / count