#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#define all(x) (x).begin(),(x).end()
//#pragma GCC optimize ("-O3")
using namespace std; void _main(); int main() { cin.tie(0); ios::sync_with_stdio(false); _main(); }
typedef long long ll; const int inf = INT_MAX / 2; const ll infl = 1LL << 60;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
//---------------------------------------------------------------------------------------------------
/*---------------------------------------------------------------------------------------------------
　　　　　　　　　　　 ∧＿∧  
　　　　　 ∧＿∧ 　（´<_｀ ）　 Welcome to My Coding Space!
　　　　 （ ´_ゝ`）　/　 ⌒i     
　　　　／　　　＼　 　  |　|     
　　　 /　　 /￣￣￣￣/　　|  
　 ＿_(__ﾆつ/　    ＿/ .| .|＿＿＿＿  
　 　　　＼/＿＿＿＿/　（u　⊃  
---------------------------------------------------------------------------------------------------*/









string S;
int N;
//---------------------------------------------------------------------------------------------------
void solve1() {
    // O(N^3)

    int ans = 0;
    rep(L, 0, N) rep(R, L, N) {
        int a = 0, b = 0;
        rep(i, L, R + 1) {
            if (S[i] == 'A') a++;
            else b++;
        }
        if (a == b) chmax(ans, R - L + 1);
    }
    cout << ans << endl;
}
//---------------------------------------------------------------------------------------------------
int A[201010], B[201010];
int get(int L, int R) {
    int res = B[R];
    if (0 < L) res -= B[L - 1];
    return res;
}
void solve2() {
    // O(N^2)

    rep(i, 0, N) {
        if (S[i] == 'A') A[i] = 1;
        else A[i] = -1;
    }
    B[0] = A[0];
    rep(i, 1, N) B[i] = B[i - 1] + A[i];
    
    int ans = 0;
    rep(L, 0, N) rep(R, L, N) {
        if (get(L, R) == 0) chmax(ans, R - L + 1);
    }
    cout << ans << endl;
}
//---------------------------------------------------------------------------------------------------
void solve3() {
    // O(N)

    rep(i, 0, N) {
        if (S[i] == 'A') A[i] = 1;
        else A[i] = -1;
    }
    B[0] = A[0];
    rep(i, 1, N) B[i] = B[i - 1] + A[i];

    map<int, int> leftest;
    leftest[0] = -1;

    int ans = 0, sm = 0;
    rep(R, 0, N) {
        sm += A[R];
        if (leftest.count(-sm)) {
            chmax(ans, R - leftest[-sm]);
        } else {
            leftest[sm] = R;
        }
    }
    cout << ans << endl;
}
//---------------------------------------------------------------------------------------------------
void _main() {
    cin >> S;
    N = S.length();

    if (N <= 100) solve1();
    else if (N <= 1000) solve2();
    else solve3();
}