#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    // S[R] - S[L] == (R - L) / 2
    // max R - L
    // 2 * S[R] - R == 2 * S[L] - L

    string s; cin >> s;
    int n = int(s.size());
    vector<int> a(n);
    rep(i,n) a[i] = (s[i] == 'A');
    vector<int> a_sum(n + 1, 0);
    rep(i,n) a_sum[i + 1] += a_sum[i] + a[i];
    map<int,vector<int>> mp;
    rep(i,n+1) mp[2 * a_sum[i] - i].push_back(i);
    int ans = 0;
    for(auto [_, v] : mp) if(v.size() >= 2u) ans = max(ans, v.back() - v[0]);
    cout << ans << endl;
}