gistfile1.m

    [[RKObjectManager sharedManager].HTTPClient postPath:@"/api/v2/json/api_users/sign_in" parameters:params success:^(AFHTTPRequestOperation *operation, id JSON)
     {
         // Success
         NSLog(@"Success");
         NSString *auth_token = [JSON class];
         NSLog(@"AUTH_TOKEN: %@", auth_token);
     } failure:^(AFHTTPRequestOperation *operation, NSError *error)
     {
         // Error
         
         if ([operation.response statusCode]  == 401) {
             UIAlertView *alert = [[UIAlertView alloc]
                                   initWithTitle:@"Failure"
                                         message:@"Failed"
                                        delegate:nil
                               cancelButtonTitle:@"OK"
                               otherButtonTitles:nil];
             [alert show];
         }
     }];