def count(a):
        global answer
        if (a.count('A') == a.count('B')):
                answer = 2*(a.count('A'))
                

        
def check(data):
        global answer
        for i in range(2*min,1,-1):
                for j in range(len(data)-i+1):
                        a = data[j:j+i]
                        count(a)
                        if(answer != 0):return
                        
                        
                        
answer = 0

data = list(input())
A = data.count("A")
B = data.count("B")
min = 0
if A >= B:min = B
else: min = A


check(data)
print(answer)