ss = input()
n = len(ss)
a = [0 for i in range(n + 1)]
b = [0 for i in range(n + 1)]
for i, s in enumerate(ss, 1):
    if s == 'A':
        a[i] = a[i - 1] + 1
        b[i] = b[i - 1]
    elif s == 'B':
        a[i] = a[i - 1]
        b[i] = b[i - 1] + 1
ans = 0
for i in range(1, n + 1):
    for j in range(0, i):
        aa = a[i] - a[j]
        bb = b[i] - b[j]
        if aa == bb and aa > ans:
            ans = aa
print(2 * ans)