S = input()
tmp = 0
from collections import defaultdict
dic = defaultdict(list)
dic[0].append(0)
for i, s in enumerate(S, start=1):
    if s == "A":
        tmp += 1
    else:
        tmp -= 1
    dic[tmp].append(i)
ans = 0
for k,V in dic.items():
    if len(V) >= 2:
        ans = max(ans,V[-1] - V[0])
print(ans)