st = list(input())
count = 0
while True:
	if (len(st) == 1) or (sorted(st) == st):
		break
	for i in range(len(st)-1):
		if st[i] == "B" and st[i+1] == "A":
			st[i] = "A"
			st[i+1] = "B"
			count += 1
print(count)