str1=input().strip()
alist = list(set(str1))
alist.sort(reverse=True)


dic1=dict()
for t in str1:
    if t not in dic1:
        dic1[t]=1
    else:
        dic1[t]=dic1[t]+1
'''
for key in dic1.keys():
    print(key,dic1[key])
'''
bb=sorted(dic1.items(), key=lambda kv: (-kv[1], -ord(kv[0])))
'''
for item in bb:
    print(item[0],item[1])
'''
print(bb[0][0])

/**************************************************************
	Problem: 1478
	User: admin
	Language: Python
	Result: Accepted
	Time:93 ms
	Memory:34480 kb
****************************************************************/