while True: try: s=input() result = {} out = [] for i in s: num = s.count(i) if num not in result.keys(): result[num] = i else: if i not in result[num]: result[num] += i
keys = sorted(result.keys(),reverse=True)
for i in keys:
out += sorted(result[i])
print(*out,sep='')
except:
break