import sys


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    s = input()
    x = False
    for k in 'abcdefghijklm':
        if set(s+k) == set('abcdefghijklm'):
            print(k)
            x = True
    if not x:
        print('Impossible')


if __name__ == '__main__':
    main()