S=input()
A=S.count("A")
B=S.count("B")
m=[A,B]
#print(min(m))
if min(m)==0:
    print(0)
else:
    for i in range(min(m)):
        
        for j in range(len(S)-2*min(m)+1+2*i):
            if len(S[j:j+2*min(m)-2*i])==2*S[j:j+2*min(m)-2*i].count("A"):
                print(len(S[j:j+2*min(m)-2*i]))
                break
            else:
                j+=1