<?php
$days = trim(fgets(STDIN));
$sleep_sum = 0;
for ( $i=0; $i<$days; $i++ ) {
	list($start, $end) = explode(" ", trim(fgets(STDIN)));
	list($start_h, $start_m) = explode(":", $start);
	list($end_h, $end_m) = explode(":", $end);
	$start_minute = ($start_h * 60) + $start_m;
	$end_minute = ($end_h * 60) + $end_m;
	if ( $start_minute > $end_minute ) {
		$end_minute += 24*60;
	}
	$sleep_sum += ($end_minute - $start_minute);
}
echo $sleep_sum.PHP_EOL;