sum = 0
gets.to_i.times do
  bedtime, wake_up_time = gets.split.map { |time| array = time.split(':').map(&:to_i) ; array[0]*60 + array[1] }
  sleeping_time = wake_up_time - bedtime
  sleeping_time += 24*60 if sleeping_time < 0
  sum += sleeping_time
end
puts sum