from collections import defaultdict

S = input()

memo = [0]*(len(S))
n = 0
for i,s in enumerate(S):
    if s == "A":
        n += 1
    else:
        n -= 1
    memo[i] = n
    
#print(S)
#print(memo)
memo = [0]+memo
    
d = defaultdict(lambda:-1)

for i,m in enumerate(memo):
    if d[m] == -1:
        d[m] = [i,0]
    else:
        d[m][1] = i
        
ans = 0
for v in d.values():
    ans = max(ans, v[1]-v[0])

print(ans)