#include <iostream>     // std::cout, std::cin
#include <string>       // std::string
#include <algorithm>    // std::transform

int main()
{

    bool answer_valid = false;
    std::string answer;

    do
    {
        std::cout << "Should the password include digits? [Y/n] ";
        std::cin >> answer;

        std::transform(answer.begin(), answer.end(), answer.begin(), ::tolower);

        answer_valid =
            (answer == "y") ||
            (answer == "n") ||
            (answer == "yes") ||
            (answer == "no");

    } while (!answer_valid);

    return 0;

}