#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define FOR(i,n,m) for(ll i=(n);i<(m);i++)
#define REP(i,n) FOR(i,0,n)
#define REPR(i,n) for(ll i=(n);i>=0;i--)
#define all(vec) vec.begin(),vec.end()
using vi=vector<int>;
using vvi=vector<vi>;
using vl=vector<ll>;
using vvl=vector<vl>;
using P=pair<ll,ll>;
using PP=pair<ll,P>;
using vp=vector<P>;
using vpp=vector<PP>;
using vs=vector<string>;
#define fi first
#define se second
#define pb push_back
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(a>b){a=b;return true;}return false;}
const ll MOD=1000000007LL;
const int INF=1<<30;
const ll LINF=1LL<<60;
int main(){
    string st;
    cin>>st;
    map<ll,ll> ma;
    ll ans=0;
    ll a=0;
    ll b=0;
    ma[0]=-1;
    REP(i,st.size()){
        if(st[i]=='A'){
            a++;
        }else{
            b++;
        }
        if(ma.find(a-b)!=ma.end()){
            chmax(ans,i-ma[a-b]);
        }else{
            ma[a-b]=i;
        }
    }
    cout<<ans<<endl;
    return 0;
}