#define _USE_MATH_DEFINES
#pragma region include
#include <iostream>
#include <iomanip>
#include <stdio.h>

#include <sstream>
#include <algorithm>
#include <iterator>
#include <cmath>
#include <complex>

#include <string>
#include <cstring>
#include <vector>
#include <bitset>

#include <queue>
#include <set>
#include <map>
#include <stack>
#include <list>

#include <ctime>
////
//#include <random>//
#pragma endregion //#include
/////////

#pragma region typedef
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
#pragma endregion //typedef
////定数
const int INF = (int)1e9;
const LL MOD = (LL)1e9+7;
const LL LINF = (LL)4e18+20;
const LD PI = acos(-1.0);
const double EPS = 1e-9;
/////////
using namespace::std;

void solve(){
	string str;
	cin>>str;
	int size = str.size();//200000=2*10^5
	int cen = 200000;
	int none = -10;
	vector<int> H(cen*2+1,none);
	H[0+cen] = -1;

	int h = 0;
	int ans = 0;
	for(int i=0;i<size;++i){
		if( str[i]=='A' ){
			++h;
		}else{
			--h;
		}
		if( H[h+cen] == none ){
			H[h+cen] = i;
		}else{
			int res = i-H[h+cen];
			ans = max(ans,res);
		}
	}
	cout << ans << endl;
}

#pragma region main
signed main(void){
	std::cin.tie(0);
	std::ios::sync_with_stdio(false);
	std::cout << std::fixed;//小数を10進数表示
	cout << setprecision(16);//小数点以下の桁数を指定//coutとcerrで別	

	solve();
}
#pragma endregion //main()