s = input()
d = {'a': 0, 'b': 0, 'c': 0, 'd': 0, 'e': 0, 'f': 0, 'g': 0, 'h': 0, 'i': 0,
     'j': 0, 'k': 0, 'l': 0, 'm': 0}

try:
    two = 0
    for i in range(len(s)):
        k = s[i:i+1]
        d[k] += 1
        if d[k] == 2:
            two += 1

    if two > 1:
        print('Impossible')
    else:
        if max(d.values()) == 1:

            for k in d.keys():
                print(k)
        elif max(d.values()) == 2:

            print(min(d, key=d.get))
        else:
            print('Impossible')
except:
    print('Impossible')