Как получить левую верхнюю и правую нижнюю широту и долготу карты в MapKit? Я использую этот код, но он нене работает должным образом. Как мне это исправить?

MKCoordinateRegion region = [map region];

double topL,topG,bottomL,bottomG;
//if latitude=55 and latitudeDelta=126 topL is 118 and it will be not at top, it will be at buttom of screen
topL = region.center.latitude + region.span.latitudeDelta/2; 

topG = region.center.longitude - region.span.longitudeDelta/2;

CLLocationCoordinate2D lt;
lt.latitude=topL;
lt.longitude=topG;
annotation = [Annotation new];
annotation.coordinate = lt;
annotation.title = @"Left";
[map addAnnotation:annotation];
[annotation release];
//if latitude=55 and latitudeDelta=126 bottomL is -7.23 and it will be not at bottom, it will be at above bottom of screen
bottomL = region.center.latitude - region.span.latitudeDelta/2;


bottomG = region.center.longitude + region.span.longitudeDelta/2;

CLLocationCoordinate2D rb;
rb.latitude=bottomL;
rb.longitude=bottomG;
annotation = [Annotation new];
annotation.coordinate = rb;
annotation.title = @"Right";
[map addAnnotation:annotation];
[annotation release];