(use gauche.collection)

(define (main args)
  (let* ([s (group-collection (string->list (read-line)))]
         [gl (map (lambda (x) (cons (length x) (car x))) s)])
    (print
     (if (= 6 (length (filter (lambda (x) (= 2 (car x))) gl)))
         (cdr (find (lambda (x) (= 1 (car x))) gl))
         "Impossible")))
  0)