#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
using namespace std;

int main(){
    string S;
    cin >> S;
    map<int, vector<int>> AB;
    int depth = 0;
    AB[depth].push_back(-1);
    for(int i = 0; i < S.size(); i++){
        if(S[i] == 'A'){
            depth++;
        }else{
            depth--;
        }
        AB[depth].push_back(i);
    }
    int ans = 0;
    for(auto i: AB){
        ans = max(ans, i.second.back() - i.second.front());
    }
    cout << ans << endl;
}