#include "bits/stdc++.h"

using namespace std;

#define DEBUG(x) cout<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cout<<#v<<":";for(int i=0;i<v.size();i++) cout<<" "<<v[i]; cout<<endl

typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define all(c) c.begin(),c.end()
const int inf = 1000000001;
const ll INF = 2e18;
const ll MOD = 1000000007;
//const ll mod = 1000000009;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p;//<<endl;
int dx[4] = { 1,0, -1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };


int main() {
	string s;
	cin >> s;
	int n = s.size();
	vi a(n + 1);
	int cnt = 0;
	int mi = inf, ma = -inf;
	rep(i, n) {
		if (s[i] == 'A') {
			cnt++;
		}
		else {
			cnt--;
		}
		a[i + 1] = cnt;
		mi = min(mi, cnt);
		ma = max(ma, cnt);
	}
	int ans = 0;
	for (int i = n; i >= 0; i--) {
		if (a[i] == 0) {
			ans = i;
			break;
		}
	}
	map<int, int> l, r;
	rep(i, n + 1) {
		if (l[a[i]] == 0) {
			l[a[i]] = i;
		}
	}
	for (int i = n; i >= 0; i--) {
		if (r[a[i]] == 0) {
			r[a[i]] = i;
		}
	}
	for (int x = mi; x <= ma; x++) {
		if (x == 0) {
			continue;
		}
		ans = max(r[x] - l[x], ans);
	}
	cout << ans << endl;
}