s = input()
l = len(s)
bef = [999999 for _ in range(2 * l + 1)]
aft = [-1 for _ in range(2 * l + 1)]

cnt = 0
bef[l] = 0
for i, ss in enumerate(s):
    if ss == "A":
        cnt += 1
    else:
        cnt -= 1
    bef[l + cnt] = min(bef[l + cnt], i)
    aft[l + cnt] = max(aft[l + cnt], i)

ans = 0
for b, a in zip(bef, aft):
    ans = max(a - b, ans)
print(ans)