<?php
$today = date_create("now");
$yesterday=date_create("-1 day");
$allSleepTimeHour=0;
$allSleepTimeMin=0;

$numDay=trim(fgets(STDIN));
for($i=1;$i<=$numDay;$i++){
  $input=explode(" ",trim(fgets(STDIN)));
  $sleepTime=explode(":",$input[0]);
  $wakeUpTime=explode(":",$input[1]);
  //日付またいで寝ているとき
  if((int)$sleepTime[0]>(int)$wakeUpTime[0]||
      ((int)$sleepTime[0]==(int)$wakeUpTime[0]&&(int)$sleepTime[1]>=(int)$wakeUpTime[0])){
    $sleepTime=date_time_set($yesterday,(int)$sleepTime[0],(int)$sleepTime[1]);
    $wakeUpTime=date_time_set($today,(int)$wakeUpTime[0],(int)$wakeUpTime[1]);
    $diff=date_diff($sleepTime,$wakeUpTime);
    $oneSleepInterval=$diff->format('%H:%I');
    $oneSleepInterval=explode(":",trim($oneSleepInterval));
    $allSleepTimeHour +=(int)$oneSleepInterval[0];
    $allSleepTimeMin +=(int)$oneSleepInterval[1];
    // $allSleepTime+=$oneSleepInterval;
  }else{
   //日付またがず寝ているとき
   // どうも、日付をまたいでいないとdate_diff使えないみたい
    $sleepTime=date_time_set($yesterday,(int)$sleepTime[0],(int)$sleepTime[1]);
    $wakeUpTime=date_time_set($today,(int)$wakeUpTime[0],(int)$wakeUpTime[1]);
    $diff=date_diff($sleepTime,$wakeUpTime);
    $oneSleepInterval=$diff->format('%H:%I');
    $oneSleepInterval=explode(":",trim($oneSleepInterval));
    $allSleepTimeHour +=(int)$oneSleepInterval[0];
    $allSleepTimeMin +=(int)$oneSleepInterval[1];
  }
}
$ans=($allSleepTimeHour*60)+$allSleepTimeMin;
echo $ans;