#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	string S;
	cin >> S;
	int N = S.size();
	vector<int> A(N + 1, 0);
	int res = 0;
	for (int i = 0; i < N; i++) {
		if (S[i] == 'A') {
			A[i + 1] = A[i] + 1;
		}
		else {
			A[i + 1] = A[i] - 1;
		}
	}
	map<int, int> mpr;
	map<int, int> mpl;
	for (int i = 0; i < N + 1; i++) {
		mpr[A[i]] = i;
	}
	for (int i = N ; i >= 0; i--) {
		mpl[A[i]] = i;
	}
	for (auto x : mpl) {
		if (mpr.count(x.first)) {
			res = max(res, mpr[x.first] - x.second);
		}
	}
	cout << res << endl;
}