N = gets.to_i
points = (1..N).map{gets.split.map(&:to_i)}

sumDist = 0
cnt = 0
(0...N).to_a.permutation{|tour|
	(1...N).each{|i| 
		sumDist += Math.sqrt((points[tour[i]][0] - points[tour[i-1]][0])**2 + (points[tour[i]][1] - points[tour[i-1]][1])**2)
	}
	cnt += 1
}

p sumDist / cnt.to_f