s = list(input())
a, b = 0, 0
ans = 0
for c in s:
  if c == "A":
    a += 1
  elif c == "B":
    b += 1

  if a > 0 and b > 0:
    if a >= b and b*2 > ans:
      ans = b * 2
    elif b >= a and a * 2 > ans:
      ans = a*2

print(ans)