#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>

using namespace std;

int main() {
	string s;
	cin >> s;

	int n = s.size();
	vector<int> starts(2*n + 1, -1);
	vector<int> ends(2*n + 1, -1);

	starts[n] = ends[n] = 0;

	int stat = 0;
	int ans = 0;
	for (int i = 0; i < s.size(); i++) {
		stat += s[i] == 'A' ? 1 : -1;
		int idx = stat + n;

		if (starts[idx] == -1) {
			starts[idx] = i + 1;
		}
		ends[idx] = i + 1;

		ans = max(ans, ends[idx] - starts[idx]);
// printf("i %d, stat %d, idx %d,  %d %d\n", i, stat, idx, starts[idx], ends[idx]);
	}

	cout << ans << endl;
	return 0;
}