# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

import random

#処理内容
def main():
	L = input()
	N = len(L)
	D = defaultdict(lambda:-1)
	D[0] = 0
	ans = 0
	val = 0
	for i in range(N):
		if L[i] == "A":
			val += 1
		else:
			val -= 1
		if D[val] != -1:
			ans = max(ans, i + 1 - D[val])
		else:
			D[val] = i + 1

	print(ans)


	
if __name__ == '__main__':
	main()