str = input()

ans = 0

for i in range(len(str)//2):
  s = (i+1)*2
  for j in range(len(str)-s+1):
    if str[j:j+s].count('A')==str[j:j+s].count('B'):
      ans = s
      break
    else:
      continue
    break
  
print(ans)