Hacker Rank: Gemstones

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        
        int numOfStones = 0;
        scanf("%d", &numOfStones);
        NSMutableArray *stoneList = [NSMutableArray arrayWithCapacity:numOfStones];
        
        for (int i=0; i<numOfStones; i++){
            
            char composition[100];
            scanf("%100s",composition);
            NSString *str = [NSString stringWithCString:composition encoding:NSUTF8StringEncoding];
            
            NSMutableSet *set = [NSMutableSet setWithCapacity:[str length]];
    
            for (int i = 0; i< str.length; i++){
                NSString *character = [str substringWithRange:NSMakeRange(i,1)];
                [set addObject:character];
            }
            
            [stoneList addObject:set];
        }
        
        NSMutableSet *commenElements = [stoneList firstObject];
        
        for (NSSet *set in stoneList){
            [commenElements intersectSet:set];
        }
        
        printf("%lu",(unsigned long)[commenElements count]);
    }
    return 0;
}