#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <iomanip>
#include <array>
#include <numeric>
#include <regex>
#include <bitset>
#include <deque>

using namespace std;
typedef long long ll;
typedef pair<int, int> p_ii;

const int INF = 1e9;
const double PI = acos(-1.0);
const ll MOD = 1e9 + 7;



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

    int n=str.size(), tn = 0, res = 0;

    map<int, int> cnt;
    for (int i = -n; i < n+1; i++) {
        cnt[i]=-1;
    }

    for (int i = 0; i < n; i++) {
        if(str[i]=='A')tn++;
        else tn--;

        if(tn == 0) res = max(res, i+1);
        if(cnt[tn]==-1)cnt[tn]=i;
        else res = max(res, i-cnt[tn]);
    }

    cout<<res<<endl;

    return 0;
}