a=list(input())
ans=0
for i in range(1,len(a)):
    j=i
    while True:
        if a[j]=='A' and a[j-1]=='B':
            a[j-1]='A'
            a[j]='B'
            ans+=1
            j-=1
            if j<=0:break
        else:
            break
print(ans)