n = gets.to_i
total = 0
n.times {
  s, e = gets.split(' ')
  sh, sm = s.split(':')
  eh, em = e.split(':')
  start = sh.to_i * 60 + sm.to_i
  finish = eh.to_i * 60 + em.to_i
  if start > finish
    finish += 24 * 60
  end
  total += finish - start
}

puts total