char s2=s[3];
    int b=s[4]-'0';
    if(s2<s1||b<a){
        return {};
    }
    else{
    for(char c=s1;c<=s2;c++){
        for(int i=a;i<=b;i++){
            v.push_back(c+std::to_string(i));
        }
    }
    }
    return v;
}