#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define REP(i, n) rep(i, 0, n)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define int long long
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
using namespace std;
typedef pair<int, int> P;
const int mod = 1000000007;
const int INF = 1e12;

vector<int> d[400010];

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    string s;
    cin >> s;
    int now = 200000;
    d[now].push_back(0);
    rep(i, 0, s.size()){
        if(s[i] == 'A'){
            now++;
            d[now].push_back(i + 1);
        }else{
            now--;
            d[now].push_back(i + 1);
        }
    }
    int ans = 0;
    rep(i, 200000 - s.size(), 200000 + s.size()){
        if(d[i].size() < 2) continue;
        int tmp = d[i][d[i].size() - 1] - d[i][0];
        chmax(ans, tmp);
    }
    cout << ans << endl;
}