#include<iostream>
#include<algorithm>
#include<map>
using ll = long long ;
using namespace std;
int main(){
	string str;
	map<ll,ll> m;
	ll ans=0,p=0;
	cin >> str;
	ll len = str.size();
	m[0] = 0;
	for(ll i=0;i<len;i++){
		if(str[i]=='A')	p++;
		else	p--;
		if(m.count(p)) ans=(ans,i-m[p]);
		else m[p]=i;
	}
	cout << ans << endl;
	return 0;
}