#yuki672
from collections import defaultdict

s=raw_input()
d=defaultdict(int)
p=0
d[0]=-1
res=0
for i in xrange(len(s)):
 if s[i]=='A':
  p+=1
 else:
  p-=1
 if p in d:
  res=max(res,i-d[p])
 else:
  d[p]=i
print res