#include<algorithm>
#include<map>
#include<iostream>
#include<vector>
using namespace std;
typedef long long lint;
typedef vector<int>vi;
typedef pair<int,int>pii;
#define rep(i,n)for(int i=0;i<(int)(n);++i)

int main(){
  string s;
  cin>>s;
  int n=s.length();
  vi acc(n+1);
  rep(i,n){
    acc[i+1]=acc[i]+(s[i]=='A'?1:-1);
  }
  int ma=0;
  map<int,vi> tt;
  rep(i,n+1){
    tt[acc[i]].push_back(i);
  }
  for(auto &e:tt){
    vi &v=e.second;
    ma=max(ma,v[v.size()-1]-v[0]);
  }
  cout<<ma<<endl;
}