from fractions import gcd
from datetime import date
from math import factorial
import functools
from heapq import*
from collections import deque
import collections
import math
from collections import defaultdict, Counter
import sys
sys.setrecursionlimit(10 ** 7)
MOD = 10 ** 9 + 7
inf = float("inf")


def make_divisors(n):
    divisors = []
    for i in range(1, int(n**0.5)+1):
        if n % i == 0:
            divisors.append(i)
            if i != n // i:
                divisors.append(n//i)

    # divisors.sort()
    return divisors


def main():
    s = list(input())
    n = len(s)
    l = [0] * (n)
    if s[0] == "A":
        l[0] = 1
    else:
        l[0] = -1
    for i in range(1, n):
        if s[i] == "A":
            l[i] = l[i-1] + 1
        else:
            l[i] = l[i - 1] - 1
    l = [0] + l

    d = defaultdict(list)
    for i in range(n+1):
        d[l[i]].append(i)

    ans = 0
    for k in d.keys():
        if len(d[k]) <= 1:
            continue
        ans = max(ans, d[k][-1] - d[k][0])
    print(ans)


if __name__ == '__main__':
    main()