N = gets.to_i
PS = (0...N).map{ gets.split.map &:to_f }
NF = (1..N).reduce(1){ |a,b| a * b } 
A = (1..N-2).reduce(1){ |a,b| a * b } * (N-1) * 2
ss = []
(0...N).each{ |i|
  x1,y1 = PS[i]
  (i+1...N).each{ |j|
	x2,y2 = PS[j]
    ss << Math.sqrt((x1-x2) ** 2 + (y1 - y2) ** 2)
  }
}
p ss.sort.reduce(:+) * A / NF