import itertools
S=input()

L = len(S)
AB = [-1]*L
for i in range(L):
    if S[i]=='A':
        AB[i] = 1
ACC = [0] + list(itertools.accumulate(AB))
ans = 0
for i in range(L+1):
    for j in range(i+ans,L+1):
        if(ACC[i]==ACC[j] and ans < j-i):
            ans = j-i
print(ans)