from collections import defaultdict

S=input()

A=[0]
for s in S:
    if s=="A":
        A.append(A[-1]+1)
    else:
        A.append(A[-1]-1)
        
D=defaultdict(list)

for i in range(len(A)):
    D[A[i]].append(i)

ANS=0

for d in D:
    ANS=max(ANS,D[d][-1]-D[d][0])

print(ANS)