S = input()

subs = set()
for bit in range(1 << len(S)):
    idx = []
    for i in range(len(S)):
        if bit & 1 << i:
            idx.append(i)
    sub = ''.join(S[i] for i in idx)
    subs.add(sub)

sub = ''
for _ in range(len(S) + 1):
    sub += 'a'
    if sub not in subs:
        print(sub)
        exit()