#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
#define dame { puts("-1"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    string s;
    cin >> s;
    int n = s.size();

    int x[500000];
    rep(i,500000)x[i] = -1;

    int cnt = 250000;
    x[cnt] = n;
    drep(i,n){
        if(s[i] == 'A'){
            cnt++;
        }else{
            cnt--;
        }
        if(x[cnt] == -1){
            x[cnt] = i;
        }
    }

    int sum[n+1] = {};
    cnt = 0;
    rep(i,n){
        if(s[i] == 'A'){
            cnt++;
        }else{
            cnt--;
        }
        sum[i+1] = cnt;
    }

    cnt = 250000;
    rep(i,n){
        if(s[i] == 'A')cnt++;
        else cnt--;
    }

    int ans = 0;

    rep(i,n){
        int tmp = cnt - sum[i];
        if(i <= x[tmp]){
            ans = max(ans, x[tmp] - i);
        }
    }

    cout << ans << endl;
    return 0;
}