#include "bits/stdc++.h" using namespace std; using ll = long long; using ld = long double; const int INF = (1 << 30) - 1; const ll INF64 = ((ll)1 << 62) - 1; const double PI = 3.1415926535897932384626433832795; const int dx[] = { 0, 1, 0, -1 }; const int dy[] = { -1, 0, 1, 0 }; int main() { ios::sync_with_stdio(false); cin.tie(0); string s; cin >> s; map mp; mp[0] = -1; int ans = 0, score = 0; for (int i = 0; i < s.size(); i++) { score += s[i] == 'A' ? 1 : -1; if (mp[score]) { ans = max(ans, i - mp[score]); } else { mp[score] = i; } } cout << ans << endl; return 0; }