Zooming out to see all the pins in MapView

I have 20-30 pins displayed on the mapview control and I want to zoom out so that all the pins are visible. Here is my code:

MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance([mp coordinate],500000, 500000); 
    [mv setReg开发者_如何学Pythonion:region animated:YES];

How can I do that?

/** 
 * Center the map on an area covering all annotations on the map. 
 */
- (void)recenterMap {
    NSArray *coordinates = [self.mapView valueForKeyPath:@"annotations.coordinate"];
    // look for the minimum and maximum coordinate
    CLLocationCoordinate2D maxCoord = {-90.0f, -180.0f};
    CLLocationCoordinate2D minCoord = {90.0f, 180.0f};
    for(NSValue *value in coordinates) {
        CLLocationCoordinate2D coord = {0.0f, 0.0f};
        [value getValue:&coord];
        if(coord.longitude > maxCoord.longitude) {
            maxCoord.longitude = coord.longitude;
        }
        if(coord.latitude > maxCoord.latitude) {
            maxCoord.latitude = coord.latitude;
        }
        if(coord.longitude < minCoord.longitude) {
            minCoord.longitude = coord.longitude;
        }
        if(coord.latitude < minCoord.latitude) {
            minCoord.latitude = coord.latitude;
        }
    }
    // create a region
    MKCoordinateRegion region = {{0.0f, 0.0f}, {0.0f, 0.0f}};
    region.center.longitude = (minCoord.longitude + maxCoord.longitude) / 2.0;
    region.center.latitude = (minCoord.latitude + maxCoord.latitude) / 2.0;
    // calculate the span
    region.span.longitudeDelta = maxCoord.longitude - minCoord.longitude;
    region.span.latitudeDelta = maxCoord.latitude - minCoord.latitude;
    // center the map on that region
    [self.mapView setRegion:region animated:YES];
}