S = input()
a = S.count("A")
ai = []
bi = []
for i in range(len(S)):
    if i < a and S[i] == "B":
        bi.append(i)
    elif i >= a and S[i] == "A":
        ai.append(i)
ans = 0
for a, b in zip(ai, bi):
    ans += b-a
print(ans)