#include <bits/stdc++.h>

using namespace std;

int main() {
    string s;
    cin >> s;
    int n = s.size();
    map<int,int> mp;
    int cnt = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
        if (s[i] == 'A') cnt++;
        else cnt--;
        if (mp.count(cnt)) {
            ans = max(ans , i - mp[cnt]);
        } else { 
            mp[cnt] = i;
        }
    }
    if (cnt == 0) {
        cout << n << endl;
    } else {
        cout << ans << endl;
    }
    return 0;
}