s=list(input())
count=0
for i in range(len(s)):
    if s[i]=="B":
        for j in range(1,len(s)-i):
            if s[i+j]=="A":
                s[i+j],s[i+j-1]=s[i+j-1],s[i+j]
                count+=1
                
print(count)