import sys
input=lambda: sys.stdin.readline().rstrip()
import collections
D=collections.defaultdict(int)
S=input()
cur=0
ans=0
for i in range(len(S)):
  if S[i]=="A":
    cur+=1
  else:
    cur-=1
  if cur==0:
    ans=i+1
  if D[cur]==0:
    D[cur]=i+1
  else:
    ans=max(ans,i+1-D[cur])
print(ans)