import itertools
S=input()
 
L = len(S)
AB = [-1]*L
ACC = [0]
cnt = 0
for i in range(L):
    if S[i]=='A':
        cnt += 1
    else:
        cnt -=1
    ACC.append(cnt)
ans = 0
for i in range(L+1):
    for j in reversed(range(i+ans,L+1)):
        if(ACC[i]==ACC[j] and ans < j-i):
            ans = j-i
            break
print(ans)