def main():
    s = [_ for _ in input()]

    for i in list(set(s)):
        if s.count(i) == 2:
            continue
        elif s.count(i) == 1:
            ans = i
        else:
            ans = "Impossible"
            break
        
    print(ans)
    
    
if __name__ == "__main__":
    main()