from collections import defaultdict

s = input()
d = defaultdict(int)
for ch in s:
    d[ch] += 1

dd = dict()
for k, v in d.items():
    # print(k, v)
    if d[k] not in dd:
        dd[v] = k
    else:
        dd[v] = dd[v] + k

for k in sorted(dd, reverse=True):
    print("".join(sorted(dd[k])), end="")