N = gets.to_i
info = readlines
num = 1
arr = []
ans = []
sum = 0

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

a = arr.combination(2).to_a
a.each do |aa|
    x = info[aa[0]-1].split(" ")[0].to_i - info[aa[1]-1].split(" ")[0].to_i
    y = info[aa[0]-1].split(" ")[1].to_i - info[aa[1]-1].split(" ")[1].to_i
    
    ans.push(Math.sqrt(x*x + y*y))
end

if N==2
    puts ans
else
    a.each do |aa|
        sum += ans[aa[0]-1] + ans[aa[1]-1]
    end
    puts sum/N
end