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

$dp = {}
def f(x)
  $dp[x] ||= if x == 0
    0
  else
    (0..x.bit_length-1).map{|a|
      if x[a] == 0
        1.0/0
      else
        l, s = LS[a]
        l /= 2 if s != a && ((1<<s)&x) != 0
        l + f(1<<a^x)
      end
    }.min
  end
end

p f(2**N-1).fdiv(2)