var inpt = readLine()!.map({$0})

func isValid() -> Bool {
    for i in 1..<inpt.count {
        if(inpt[i-1] == "B" && inpt[i] == "A") {
            return false
        }
    }
    return true
}

var cnt = 0
while (!isValid()) {
    for i in 1..<inpt.count {
        if inpt[i-1] == "B" && inpt[i] == "A" {
            inpt[i-1] = "A"
            inpt[i] = "B"
            cnt += 1
            break
        }
    }
}

print(cnt)