s = input()

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

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

if s[l:r] == "AA":
    cnt = 1
    surplus = "A"
elif s[l:r] == "BB":
    cnt = 1
    surplus = "B"
else:
    cnt = 0
    surplus = ""

while l < len_s and r < len_s:
    # s[l:r] に含まれるAの個数とBの個数が異なるとき
    if cnt > 0:
        if s[l] != s[r]:
            if s[r] == surplus:
                cnt += 1
            else:
                cnt -= 1

            if cnt == 0:
                surplus = ""
        l += 1
        r += 1
        continue

    # s[l:r] に含まれるAの個数とBの個数が等しいとき
    if l - 2 >= 0:
        if s[l - 2:l] == "AB" or s[l - 2:l] == "BA" == 1:
            l -= 2
            cnt = 0
            continue

    if l - 1 >= 0 and r < len_s:
        if (s[l - 1] + s[r]) == "AB" or (s[l - 1] + s[r]) == "BA":
            l -= 1
            r += 1
            cnt = 0
            continue

    if r + 2 <= len_s:
        if s[r:r + 2] == "AB" or s[r:r + 2] == "BA":
            r += 2
            cnt = 0
            continue

    # ひとつ右にずらす
    if s[l] != s[r]:
        cnt = 1
        if s[r] == "A":
            surplus = "A"
        else:
            surplus = "B"
    l += 1
    r += 1

print(r - l)