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

int main(){
    string s;
    cin >> s;

    map<int, int> mp;
    mp[0] = -1;
    int now = 0;
    int ans = 0;
    for(int i = 0;i < s.size();i++){
        now += s[i] == 'A' ? 1 : -1;
        if(mp.find(now) != mp.end()){
            ans = max(ans, i-mp[now]);
        }else{
            mp[now] = i;
        }
    }

    cout << ans << endl;
    return 0;
}