#include <iostream>
#include <iomanip>
#include <cstring>
#include <algorithm> 
#include <math.h>
#include <queue>
#include <functional>
#include <map>
#include <vector>
#include <string>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;
int dx[] = { 0, 1, 1, 1, 0, -1, -1, -1 };
int dy[] = { -1, -1, 0, 1, 1, 1, 0, -1 };
const ll MOD = 1000000007;
const ll INF = 1000000000;
const int MAX = 10000000;





int main() {

    string s;
    cin >> s;
    map<int, int> pos;
    map<int, bool> exist;

    int count = 0;
    exist[count] = true;
    pos[count] = -1;
    int ans = 0;
    for (int i = 0; i < s.size(); i++){
        if (s[i] == 'A') count++;
        else count--;
        if (exist[count]){
            ans = max(ans, i - pos[count]);
        }
        else{
            pos[count] = i;
            exist[count] = true;
        }
    }

    cout << ans << endl;



}