Adding numbers in an Array with a given condition

I have numbers in an array, odd and even, I have to add the odd ones with one another and the even ones wit one another. I am very confused as to how to go about this because of the parameters and conditions given:

In my Adder.h file I have the following:

@interface ConditionalAdder : NSObject

- (instancetype)initWithNumbers:(NSArray *)numbers;

- (int)sumWithCondition:(NSString *)condition;

@end

In my Main.m file I have the following code:

#import "ConditionalAdder.h"

int main(int argc, const char * argv[]) {
  @autoreleasepool {

    ConditionalAdder *adder1 = [[ConditionalAdder alloc] 
    initWithNumbers:@[@1, @2, @3, @4, @5]];
    NSLog(@"%i", [adder1 sumWithCondition:@"even"]);
    NSLog(@"%i", [adder1 sumWithCondition:@"odd"]);

    ConditionalAdder *adder2 = [[ConditionalAdder alloc] 
    initWithNumbers:@[@13, @88, @12, @44, @99]];
    NSLog(@"%i", [adder2 sumWithCondition:@"even"]);

    ConditionalAdder *adder3 = [[ConditionalAdder alloc] 
    initWithNumbers:@[]];
    NSLog(@"%i", [adder3 sumWithCondition:@"odd"]);
  }

  return 0;
}

I know that this method:

- (int)sumWithCondition:(NSString *)condition;

Should return an integer, but what string am I supposed to pass through the parameter?