#include<iostream>
#include<algorithm>
#include<vector>
using ll = long long ;
using namespace std;
int main(){
	string str;
	ll ans=0;
	cin >> str;
	ll len = str.size();
	vector<ll> sum(len);
	if(str[0]=='A')	sum[0]++;
	else	sum[0]--;
	for(ll i=1;i<len;i++){
		if(str[i]=='A')	sum[i]=sum[i-1]+1;
		else	sum[i]=sum[i-1]-1;
	}
	for(ll i=0;i<len;i++){
		for(ll j=i+1;j<len;j++){
			if(sum[j]==sum[i])	ans = max(j-i,ans);
		}
	}
	cout << ans << endl;
	return 0;
}