a=input()
a=a.upper()
b=[]
for i in range(26):
    b.append(0)
for i in range(len(a)):
    for j in range(65,65+26):
        if a[i]==chr(j):
            b[j-65]+=1
sum=b[0]
ans=[]
for i in range(1,26):
    if sum<=b[i]:
        sum=b[i]
        ans.append(chr(i+65))
if sum==b[0]:
    ans.append('A')
print(ans)