arg = list(input())
count = 0
i = 0
while(i < len(arg)-1):
	if arg[i] == 'B' and arg[i+1] == 'A':
		arg[i] = 'A'
		arg[i+1] = 'B'
		count += 1
		if i > 0 and arg[i-1] == 'B':
			i -= 2
	i += 1
print(count)