char = input().upper()
A = ord('A')
max = 0
max_coord = 0
alph = [0 for i in range(26)]

for i in char:
    for j in range(0, 26):
        if A+j == ord(i):
            alph[j] = alph[j]+1

for i in range(26):
    if alph[i] == 0:
        continue
    elif alph[i] == max:
        max = -1
    elif alph[i] > max:
        max = alph[i]
        max_coord = int(i)

if max < 0:
    print("?")
else:
    print(chr(ord('A')+max_coord))