#include <bits/stdc++.h>
#include <string>
#define ft first
#define sc second
#define pt(sth) cout << sth << "\n"
#define chmax(a, b) (a)=max(a, b)
#define chmin(a, b) (a)=min(a, b)
#define moC(a, s, b) (a)=((a)s(b)+MOD)%MOD
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
static const ll INF=1e18;
static const ll MAX=101010;
static const ll MOD=1e9+7;

/*
 for(i=0; i<N; i++)
   cin >> a[i];
*/



int main(void) {
  ll i, j, k;
  
  string s;
  cin >> s;
  ll N=s.size();
  s="z"+s;
  ll ans=0;
  
  map<ll, vector<ll>> mp;
  mp[0].push_back(0);
  ll t=0;
  for(i=1; i<=N; i++) {
    ll r=(s[i]=='A' ? 1:-1);
    t+=r;
    
    mp[t].push_back(i);
  }
  
  for(auto it=mp.begin(); it!=mp.end(); it++) {
    ll S=it->sc.size();
    chmax(ans, it->sc[S-1]-it->sc[0]);
  }
  
  pt(ans);
}