class Calc0178
  def initialize(args)
    args = args.map { |l| l.chomp.split(/\s+/) }
    @n = args.shift.first.to_i
    @lines = args.map { |l| l.map(&:to_i) }
  end

  def run
    ws = @lines.map { |l| l[0] + l[1] * 4 }
    max = ws.max
    ws = ws.map { |l| max - l }
    if ws.any?(&:odd?)
      -1
    else
      ws.inject(:+) / 2
    end
  end
end

puts Calc0178.new(STDIN.readlines).run if __FILE__ == $0