#include<bits/stdc++.h>
using namespace std;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    string S;
    cin >> S;
    int N = S.size();
    int n = N + 5;
    vector<int> pos(n * 2, -1);
    int cum = n;
    pos[cum] = 0;
    int ans = 0;
    for (int p = 0; p < N; ++p){
        if (S[p] == 'A'){
            ++cum;
        }else{
            --cum;
        }
        if (pos[cum] == -1){
            pos[cum] = p + 1;
        }else{
            ans = max(ans, p + 1 - pos[cum]);
        }
    }
    cout << ans << endl;
    return 0;
}