S = input()
ans = 0
if len(set(S)) != 1:
    s = list(S)
    for i in range(len(s)):
        if s[i] == "A":
            j = i
            while s[i-1]=="B":
                i-=1
                ans+=1
            temp=s[i]
            s[i]=s[j]
            s[j]=temp
print(ans)