The "sum of all persons" would be to fetch all persons and count them
int sum = fetchedObjects.count;
But perhaps you mean the "sum of the prices of all gifts of all persons". If you think about it, it is the same as the sum of the prices of all gifts. Thus, you can just fetch the gifts and calculate
NSNumber *sum = [self.fetchedResultsController.fetchedObjects
valueForKeyPath:@"@sum.priceOfGift"];