Nested array sum

@interface Solution: NSObject
-(int)arrayValue:(NSArray *)arr;
@end

@implementation Solution
-(int)arrayValue:(NSArray *)arr {
    return [self pfx_arrayValue:arr depth:1];
}

-(int)pfx_arrayValue:(NSArray *)arr depth:(int)d {
    int sum = 0;
    for(id element in arr) {
        if([element isKindOfClass:[NSNumber class]]) {
            NSNumber *num = (NSNumber *)element;
            sum += num.intValue * d;
        } else {
            sum += [self pfx_arrayValue:(NSArray *)element depth:d+1];
        }
    }
    return sum;
}
@end