REFERENCES:https://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSNumber_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40003704
REFERENCES:https://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSNumber_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40003704