int alternating(const string& str, char a, char b) {
    char prev = 0;
    int len = 0;
    for (char c : str) {
        if (c != a && c != b) continue; 
        if (c == prev) return -1;       
        prev = c;
        len++;
    }
    return len;
}

int alternate(string s) {
    set<char> str_set(s.begin(), s.end());
    vector<char> uniques;
    
    copy(str_set.begin(), str_set.end(), std::back_inserter(uniques));
    int bgs = 0;
    
    for(int i = 0; i<uniques.size(); i++){
        for (int j = i+1; j<uniques.size(); j++) {
            int len = alternating(s, uniques[i], uniques[j]);
            if(len>0 && len>bgs){
                bgs = len;
            }
        } 
    }
    
    return bgs;
}