n = gets.to_i
sum = 0

n.times do
  (h_s, m_s), (h_w, m_w) = gets.split.map { |e| e.split(':').map(&:to_i) }
  sum += ((h_w * 60 + m_w) - (h_s * 60 + m_s)) % 1440
end

puts sum