#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

//#define int long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define all(c) begin(c),end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1'000'000'007;
template<class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	string s; cin >> s;
	int n = s.size();
	vector<int> v(n + 1);
	rep(i, 0, n) {
		v[i + 1] = v[i] + (s[i] == 'A' ? +1 : -1);
	}
	dump(v);
	unordered_map<int, int> mini;
	unordered_map<int, int> maxi;
	int ans = 0;
	rep(i, 0, n + 1) {
		if (!mini.count(v[i])) {
			mini[v[i]] = i;
			maxi[v[i]] = i;
		}
		else {
			chmin(mini[v[i]], i);
			chmax(maxi[v[i]], i);
		}
		chmax(ans, maxi[v[i]] - mini[v[i]]);
	}
	dump(maxi, mini);
	cout << ans << endl;
	return 0;
}