#include <bits/stdc++.h>
#define rep(i,n)for(int i=0;i<(n);i++)
#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3f
#define EPS (1e-10)
using namespace std;
typedef long long ll;
typedef pair<int,int>P;

signed main(){
	string s;cin>>s;
	map<int,int>mp;
	int a=0,b=0;
	mp[a-b]=-1;
	int ans=0;
	rep(i,s.size()){
		if(s[i]=='A')a++;
		else b++;
		if(mp.count(a-b)){
			ans=max(ans,i-mp[a-b]);
		}
		else mp[a-b]=i;
	}
	cout<<ans<<endl;
}