n = gets.to_i

x = []
y = []

n.times {
	x1, y1 = gets.split.map &:to_i
	x << x1
	y << y1
}

len = []

(0...n).to_a.permutation(n).each{ |pp|
	sum = 0

	pp.each_cons(2){ |a,b|
		sum += ((x[a] - x[b]) ** 2.0 + (y[a] - y[b]) ** 2.0) ** 0.5
	}
	
	len << sum
}

puts len.inject(:+) / len.size
