(require 'clojure.set)
(defn Not-Found [S]
  (let [S' (set S)]
    (if (= 26 (count S'))
      "None"
      (-> #{\a \b \c \d \e \f \g \h \i \j \k \l \m \n \o \p \q \r \s \t \u \v \w \x \y \z}
        (clojure.set/difference S')
        sort
        first))))



(-> (read-line)
  Not-Found
  println)