gets
puts readlines.inject(0) { |sum, line|
  start_time, end_time = line.chomp.split
  start_hour, start_min = start_time.split(':').map(&:to_i)
  end_hour, end_min = end_time.split(':').map(&:to_i)

  diff_hour = (end_hour - start_hour) % 12
  diff_min = (end_min - start_min).abs
  if start_min > end_min
    diff_hour -= 1
    diff_min = 60 - diff_min
  end

  diff_hour = 0 if diff_hour.negative?
  sum + diff_hour * 60 + diff_min
}