S = input()
tmp = 0
dic = {}
dic[0] = 0
ans = 0
for i, s in enumerate(S, start=1):
    if s == "A":
        tmp += 1
    else:
        tmp -= 1
    if tmp in dic:
        ans = max(ans,i - dic[tmp])
    else:
        dic[tmp] = i
print(ans)