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)
    
first = defaultdict(int)
last = defaultdict(int)

for i,m in enumerate(memo):
    if first[m]:
        last[m] = i+1
    else:
        first[m] = i+1
        
ans = 0
for k,v in first.items():
    if last[k]:
        ans = max(ans, last[m]-first[m])

if last[0]:
    ans = max(ans,last[0])

print(ans)