def main():
    s = [_ for _ in input()]
    
    cnt = 0
    for i in list(set(s)):
        if s.count(i) == 2:
            continue
        elif s.count(i) == 1:
            ans = i
            cnt += 1
            if cnt > 1:
                ans = "Impossible"
                break
        else:
            ans = "Impossible"
            break
        
    print(ans)
    
    
if __name__ == "__main__":
    main()