#include <iostream>
#include <fstream>
#include <string>
#include <cstring> 
#include <cmath>  
#include <cstdlib>
#include <ctime>
#include <vector>
#include <algorithm>
#include <numeric>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>
#include <functional>
#include <cctype>
#include <list>
//#include <boost/multiprecision/cpp_int.hpp>

#define BIT(a) (1 << (a))

using namespace std;
//using namespace boost::multiprecision;

long long MOD = 1000000007;


long long mod_pow(long long x, long long n){
    long long res = 1;
    for(int i = 0;i < 60; i++){
        if(n >> i & 1) res = res * x % MOD;
        x = x * x % MOD;
    }
    return res;
}

char s[200005];

int main(void){
    cin >> s;
    int len = strlen(s);
    int Acnt = 0;
    int Amax = 0;
    int Bcnt = 0;
    int Bmax = 0;
    int ans = 0;
    for (int i = 0; i < len; i++){
        if (s[i] == 'A'){
            Acnt++;
            Amax = Amax<Acnt?Acnt:Amax;
        }
        if (s[i] == 'B'){
            if (Acnt > 0) --Acnt;
            ans = ans<Amax-Acnt?Amax-Acnt:ans;
        }
    }
    for (int i = 0; i < len; i++){
        if (s[i] == 'B'){
            Bcnt++;
            Bmax = Bmax<Bcnt?Bcnt:Bmax;
        }
        if (s[i] == 'A'){
            if (Bcnt > 0) --Bcnt;
            ans = ans<Bmax-Bcnt?Bmax-Bcnt:ans;
        }
    }
    cout << ans*2 << endl;
    return 0;
}