import sequtils,strutils
var
    S = stdin.readline
    L = S.len
    cnt : array[-200001..200001, int]
    n : int
    ans = 0
for i,c in S:
    if c == 'A':
        n += 1
    else:
        n -= 1
    if n == 0:
        ans = i + 1
    elif cnt[n] == 0:
        cnt[n] = i + 1
    else:
        ans = max(i - cnt[n] + 1,ans)
echo ans