word = list(input().upper()); abclist = {}
if 1 <= len(word) <= 1000000:
for n in word:
if 65<=ord(n)<=90:
try: abclist[n] += 1
except: abclist[n] = 1
abcmax = [val for val in abclist.values()]; max_value = max(abcmax); cnt = 0
for n in range(len(abclist.values())):
if abcmax[n] == max(abcmax):
cnt+=1
if cnt == 1:
print(word[abcmax.index(max_value)])
else:
print('?')
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