s = input().strip()

counts = {}
for c in 'abcdefghijklmnopqrstuvwxyz':
    counts[c] = 0
for ch in s:
    counts[ch] += 1

candidates = []
for c in 'abcdefghijklmnopqrstuvwxyz':
    k = counts[c]
    candidates.append(c * (k + 1))

def is_subsequence(t, s):
    it = 0
    for ch in t:
        pos = s.find(ch, it)
        if pos == -1:
            return False
        it = pos + 1
    return True

valid = []
for candidate in candidates:
    if not is_subsequence(candidate, s):
        valid.append(candidate)

print(min(valid) if valid else '')