#!ruby -rtsort

class Array
  include TSort
  alias tsort_each_node each_index
  def tsort_each_child(node)
    yield self[node]
  end
end

N=gets.to_i
L=[]
S=[]
N.times.map{
  a, b = gets.split.take(2).map(&:to_i)
  L << a
  S << b-1
}

used = []
p S.strongly_connected_components.map{|a|
  t = a.min_by{|i|L[i]}
  n = a.map{|i|
    if i != t || used[S[i]]
      L[i]/2.0
    else
      L[i]
    end
  }.sum.tap{
    a.each{|i|
      used[i] = true
    }
  }
}.sum*1.0