int base;
for(int i=0; i<s.size(); i++){
        if(isalpha(s[i])){
                if(islower(s[i])){
                        base = 'a';
                }else{
                        base = 'A';
                }
                s[i] = ((s[i] - base + k) % 26) + base;
        }
}
return s;

}