x;
main(a,b,c,d)
{
	for(scanf("%*d\n");~scanf("%d:%d %d:%d\n",&a,&b,&c,&d);x+=c-a)
	{
		a*=60,c*=60,a+=b,c+=d;
		if(a>c)c+=1440;
	}
	printf("%d\n",x);
}