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
  diff_hour = 24 + diff_hour if diff_hour.negative?
  diff_min = end_min - start_min
  if diff_min.negative?
    if diff_hour.zero?
      diff_hour = 23
    else
      diff_hour -= 1
    end

    diff_min = 60 + diff_min
  end

  sum + (diff_hour % 24) * 60 + diff_min
}