static String unhide_letter(String word, String hidden_word, Character letter){
    Integer dlugosc = word.length();
    for(Integer i=0; i<dlugosc; i++){
      if(letter.toUpperCase(letter) == word.charAt(i).toUpperCase(word.charAt(i))){
        hidden_word = hidden_word.substring(0, i) + word.charAt(i) + hidden_word.substring(i+1);
      }
    }
    return hidden_word;