ss = input()
n = len(ss)
sss = [0 for _ in range(n + 1)]
m = {0: 0}
for i, s in enumerate(ss, 1):
    if s == 'A':
        sss[i] = sss[i - 1] + 1
    else:
        sss[i ] = sss[i - 1] - 1
    if not sss[i] in m:
        m[sss[i]] = i
ans = 0
for i in range(1, n + 1):
    ans = max(ans, i - m[sss[i]])
print(ans)