def func(s):
    n = len(s)
    max_len = 0
    count = {0: -1}
    balance = 0

    for i in range(n):
        if s[i] == "A":
            balance += 1
        else:
            balance -= 1

        if balance in count:
            max_len = max(max_len, i - count[balance])
        else:
            count[balance] = i
            
    return max_len

s = input()
print(func(s))