#include <iostream>
#include <cstdint>

using namespace std;

int main()
{
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	
	static char S[200001];
	cin >> S;

	static uint32_t first_index[400001], ans = 0, count_A = 0, count_B = 0, i;
	for (i = 0; i <= 400001; ++i)
		first_index[i] = UINT32_MAX;

	first_index[0] = 0;
	for (i = 0; ; ++i)
	{
		switch (S[i])
		{
		case 'A':
			++count_A;
			break;

		case 'B':
			++count_B;
			break;

		case '\0':
			cout << ans << '\n';
			return 0;
		}

		if (first_index[200000 + count_A - count_B] == UINT32_MAX)
			first_index[200000 + count_A - count_B] = i + 1;
		else if (ans < i + 1 - first_index[200000 + count_A - count_B])
			ans = i + 1 - first_index[200000 + count_A - count_B];
	}

	return -1;
}