#include <bits/stdc++.h>
using namespace std;
#define i64 long long
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)

int main(){
    string s;
    cin >> s;
    vector<int> a(s.size());
    rep(i,0,s.size() - 1){
        if(s[i] == 'A') a[i] = 1;
        else a[i] = -1;
    }
    rep(i,1,s.size() - 1){
        a[i] += a[i - 1];
    }

    set<int> st;
    int ans = 0;
    map<int,int> mp;
    mp[0] = -1;
    rep(i,0,s.size() - 1){
        if(mp.count(a[i])){
            ans = max(ans , i - mp[a[i]]);
        }
        else{
            mp[a[i]] = i;
        }
    }

    cout << ans << endl;

}