#include<bits/stdc++.h>
using namespace std;

int main(){
	cin.tie(0),ios::sync_with_stdio(false);
	string s; cin>>s;
	vector<int>v(s.size()+1); v.front()=0;
	unordered_map<int,int>mx,mn;
	set<int>st;
	for(int i=0;i<s.size();++i){
		v.at(i+1)=v.at(i);
		if(s.at(i)=='A')++v.at(i+1);
		else --v.at(i+1);
		mx[v.at(i+1)]=0;
		mn[v.at(i+1)]=1e9;
		st.insert(v.at(i+1));
	}
	for(int i=1;i<v.size();++i){
		mx[v.at(i)]=max(i,mx[v.at(i)]);
		mn[v.at(i)]=min(i,mn[v.at(i)]);
	}
	int ans=0;
	for(auto&&i:st)ans=max(ans,mx[i]-mn[i]);
	cout<<ans<<"\n"s;
}