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