#include <string>
#include <iostream>

std::string reverse_letter(const std::string &str)
{
    std::string alphabet ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ");
    std::string result ("");
    for (int i = str.length() - 1; i >= 0; i--){
        if (alphabet.find(str[i]) != std::string::npos){
            result.push_back(str[i]);
        }
    }
    return result;
}

int main(){
    std::cout << reverse_letter("Ba3rti1bo0y!");
}