#include<stdio.h>
#include <string>
#include <cctype>

int main() 
{
    std::string numeric_string;
    std::cin >> numeric_string;

    std::string decoded_message = "";
    bool valid = true;

    for (size_t i = 0; i < numeric_string.length(); i += 2)
    {
        std::string pair_str = numeric_string.substr(i, 2);

        int ascii_value = std::stoi(pair_str);

        if ((ascii_value >= 65 && ascii_value <= 90) || (ascii_value >= 97 && ascii_value <= 122))
        {
            decoded_message += static_cast<char>(ascii_value);
        } 
        else 
        {
            valid = false;
            break;
        }
    }
    if (valid)
    {
        std::cout << decoded_message << std::endl;
    } 
    else
    {
        std::cout << "Invalid input" << std::endl;
    }
    return 0;
}