#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 cnt[400005];

int main(void){
    cin >> s;
    int len = strlen(s);
    int Acnt = 0;
    int ans = 0;
    memset(cnt, -1, sizeof(cnt));
    cnt[200000] = 0;
    for (int i = 0; i < len; i++){
        if (s[i] == 'A'){
            ++Acnt;
        }
        if (s[i] == 'B'){
            --Acnt;
        }
        if (cnt[Acnt+200000] == -1){
            cnt[Acnt+200000] = i+1;
        }else{
            if (ans < i+1-cnt[Acnt+200000]) ans = i+1 - cnt[Acnt+200000];
        }
    }
    cout << ans << endl;
    return 0;
}