word = input().strip().upper()

count = {}

for i in word:
    if i in count:
        count[i] += 1
    else:
        count[i] = 1

max_value = max(count.values())
max_count = 0
answer = ''

for ch in count:
    if count[ch] == max_value:
        max_count += 1
        answer = ch

if max_count > 1:
    print('?')
else:
    print(answer)