(use srfi-13)

(define (main args)
  (let ([s (read-line)]
        [tm (alist->tree-map (map cons (string->list "abcdefghijklm") (make-list 13 0)))])
    (string-for-each (lambda (c)
                       (cond [(char<=? #\a c #\m) (tree-map-update! tm c (cut + <> 1))]
                             [else #f])) s)
    (let ([n (length (filter (cut = <> 0) (tree-map-values tm)))])
      (cond [(zero? n) (for-each print (tree-map-keys tm))]
            [(= 1 n) (tree-map-for-each tm (lambda (k v) (if (zero? v) (print k))))]
            [else (print "Impossible")])))
  0)