def solve():
    s = input()
    cntA = s.count("A")

    ans1 = 0
    s1 = s[:]
    while s1[:cntA].count("A") != cntA:
        bl = s1.rfind("B")
        ar = s1.rfind("A")
        ans1 += len(s1) - bl - 1
        s1 = s1[:bl] + s1[bl+1:ar+1] 

    print(ans1) 


if __name__=="__main__":
    solve()