from collections import defaultdict


S = input()
cnt = [0] * (len(S) + 1)
for i, c in enumerate(S):
    if c == 'A':
        cnt[i + 1] = cnt[i] + 1
    else:
        cnt[i + 1] = cnt[i] - 1
idx = defaultdict(list)
for i, v in enumerate(cnt):
    idx[v].append(i)
ans = 0
for k, v in idx.items():
    ans = max(ans, v[-1] - v[0])
print(ans)