#include "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;

const int INF = (1 << 30) - 1;
const ll INF64 = ((ll)1 << 62) - 1;
const double PI = 3.1415926535897932384626433832795;

const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { -1, 0, 1, 0 };

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	string s;
	cin >> s;

	map<int, int> mp;
	mp[0] = -1;

	int ans = 0, score = 0;
	for (int i = 0; i < s.size(); i++) {
		score += s[i] == 'A' ? 1 : -1;
		if (mp.count(score)) {
			ans = max(ans, i - mp[score]);
		} else {
			mp[score] = i;
		}
	}

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