s = input()

if s.count("A") == 0 or s.count("B") == 0:
    print(0)
    exit()

l = 0
r = 2
len_s = len(s)

while l < len_s and r < len_s:
    if s[l:r].count("A") != s[l:r].count("B"):
        l += 1
        r += 1
        continue

    if l - 1 >= 0 and r + 1 <= len_s:
        if (s[l - 1] + s[r + 1]).count("A") == 1 and \
            (s[l - 1] + s[r + 1]).count("B") == 1:
            l -= 1
            r += 1
            continue

    if r + 2 <= len_s:
        if s[r:r + 2].count("A") == 1 and \
            s[r:r + 2].count("B") == 1:
            r += 2
            continue

    l += 1
    r += 1

print(r - l)