Loop if Invalid input

[jwoot001]

Hi, Im a beginner with c++, I would like to ask for help with my loop thingy i dont know where to put the loop but the program should be like this

CONVERSION                                                                                                                                                                      
                                                                                                                                                                                
Base 11 to Decimal                                                                                                                                                              
Base 11: 123456789AB                                                                                                                                                            
Invalid Input! 
Base 11: BBBBB
Invalid Input!
Base 11: A
Decimal: 10

#include <iostream>
#include <string>
using namespace std;

int main ()
{
string str;
cout <<"CONVERSION\n\n";
cout <<"Base 11 to Decimal\n";
cout << "Base 11: ";
getline (std::cin,str);
const auto bad_loc = str.find_first_not_of("0123456789aA");
if(bad_loc != std::string::npos) {
    std::cerr << "Invalid Input!\n";
    return 1;
}
unsigned long ul = std::stoul (str,nullptr,11);
cout << "Decimal: " << ul << '\n';
return 0;
}

where shoul i put the loop ?