word=input().lower()
result=[0]*26
alpabet=['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
for a in range(len(word)):
    for i in range(26):
        if alpabet[i]==word[a]:
            result[i]=result[i]+1
m=max(result)
if result.count(m) > 1:
    print('?')
else:
    print(alpabet[result.index(m)].upper())