#include <bits/stdc++.h>
using namespace std;

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) for (int i = (n) - 1; i >= 0; i--)
#define ALL(v) (v).begin(), (v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x, n) bitset<n>(x)
#define PI 3.14159265358979323846
 
typedef long long ll;
typedef pair< int, int > P;   
typedef pair< ll, P > PP;

//-----------------------------------------------------------------------------

string s;
int cnt[200010], ans;
map<int, P> mp;

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

	cin >> s;
	mp[0] = P(0, -1);
	REP(i, (int)s.size()) {
		if (s[i] == 'A') cnt[i + 1] = cnt[i] + 1;
		else cnt[i + 1] = cnt[i] - 1;
		if (mp.find(cnt[i + 1]) == mp.end()) mp[cnt[i + 1]] = P(i + 1, -1);
		else mp[cnt[i + 1]].se = i + 1, ans = max(ans, mp[cnt[i + 1]].se - mp[cnt[i + 1]].fi);
	}
	cout << ans << endl;

    return 0;
}