numOps :: String -> Int
numOps = numOps' 0 0
    where
        numOps' a _ [] = a
        numOps' a b (c : cs)
            | c == 'A' = numOps' (a + b) b cs
            | c == 'B' = numOps' a (b + 1) cs
            | otherwise = numOps' a b cs

main :: IO ()
main = print . numOps =<< getLine