N = gets.to_i
P = readlines.map {|s| s.split.map(&:to_i) }

def calc(x1,y1,x2,y2)
  Math.sqrt((x1-x2)**2+(y1-y2)**2)
end

n = 0
pt = [*0...N].permutation
pt.each do |a|
  a.each_cons(2) {|i,j| n += calc(*P[i],*P[j]) }
end
puts n / pt.size