ior = io.read
local str = ior()
local left = {}
local right = {}
left[0] = 0
local n = #str
local cur = 0
for i = 1, n do
  if(str:sub(i, i) == "A") then
    cur = cur + 1
  else
    cur = cur - 1
  end
  if(left[cur] == nil) then
    left[cur] = i
  else
    right[cur] = i
  end
end
local max = 0
for k, v in pairs(left) do
  if(right[k] ~= nil) then
    max = math.max(right[k] - v, max)
  end
end
print(max)