import sys
input_str = sys.stdin.readline().rstrip().upper()
alphabet_count = [0] * 26
for char in input_str:
if 'A' <= char <= 'Z':
index = ord(char) - ord('A')
alphabet_count[index] += 1
max_count = max(alphabet_count)
most_frequent_alphabet = chr(ord('A')+ alphabet_count.index(max_count))
if alphabet_count.count(max_count) >= 2:
print("?")
else:
print(most_frequent_alphabet)