n = gets.to_i
a = readlines.map{|e| e.split.map &:to_f}
count = 0.0
sum = 0
[*0..n-1].permutation.each{ |e|
    e.each_cons(2) { |i, j|
        x = a[i][0] - a[j][0]
        y = a[i][1] - a[j][1]
        sum += Math.sqrt(x ** 2 + y ** 2)
    }
    count += 1
}
puts sum / count