#include<deque>
#include<queue>
#include<vector>
#include<algorithm>
#include<iostream>
#include<set>
#include<cmath>
#include<tuple>
#include<string>
#include<chrono>
#include<functional>
#include<iterator>
#include<random>
#include<unordered_set>
#include<array>
#include<map>
#include<iomanip>
using namespace std;
typedef long long int llint;
typedef long double lldo;
#define mp make_pair
#define mt make_tuple
#define pub push_back
#define puf push_front
#define pob pop_back
#define pof pop_front
#define fir first
#define sec second
#define res resize
#define ins insert
#define era erase
//cout<<setprecision(20)
const llint mod=1000000007;
const llint big=2.19e15+1;
const long double pai=3.141592653589793238462643383279502884197;
const long double eps=1e-15;
template <class T,class U>void mineq(T& a,U b){if(a>b){a=b;}}
template <class T,class U>void maxeq(T& a,U b){if(a<b){a=b;}}
llint gcd(llint a,llint b){if(a%b==0){return b;}else return gcd(b,a%b);}
llint lcm(llint a,llint b){return a/gcd(a,b)*b;}
template<class T> void SO(T& ve){sort(ve.begin(),ve.end());}
template<class T> void REV(T& ve){reverse(ve.begin(),ve.end());}
int LBI(vector<lldo>&ar,lldo in){return lower_bound(ar.begin(),ar.end(),in)-ar.begin();}
int UBI(vector<lldo>&ar,lldo in){return upper_bound(ar.begin(),ar.end(),in)-ar.begin();}
int main(void){
	cin.tie(0);
	ios::sync_with_stdio(false);
	string str;cin>>str;
	int i,n,ans=0,g=0;n=str.size();
	vector<int>dp(2*n+1,mod);
	dp[n]=0;g=n;
	for(i=0;i<n;i++){
		if(str[i]=='A'){g++;}
		else{g--;}
		maxeq(ans,i-dp[g]);
		mineq(dp[g],i);
	}
	cout<<ans<<endl;
	return 0;
}