#include <cassert>
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
using namespace::std;
typedef long long LL;
typedef pair<int, int> PI;
typedef pair<LL, LL> PL;

int main(){
    string s;
    cin >> s;
    vector<int> ss(s.size());
    vector<vector<int>> cnt(2*s.size()+10);
    for(int i = 0;i < s.size();i++){
        if(s[i] == 'A') ss[i] = 1;
        else    ss[i] = -1;
    }
    int count = s.size();
    for(int i = s.size()-1;i >= 0;i--){
        count += ss[i];
        cnt[count].push_back(i);
    }
    int ans = 0;
    for(int i = 0;i < cnt.size();i++){
        if(cnt[i].size() == 0)  continue;
        else if(i == s.size())  ans = max(ans, (int)s.size()-cnt[i][0]);
        else if(cnt[i].size() > 1)  ans = max(ans, cnt[i][0]-cnt[i][cnt[i].size()-1]);
    }
    cout << ans << endl;
}