#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1LL << 30;
int MOD = 1e9+7;
main(){
    string S;
    cin >> S;
    const int MAX_N = 200000;
    int cnt = MAX_N;
    vector<int> A(MAX_N*2 + 1,INF),B(MAX_N*2 + 1);
    A[cnt] = 0,B[cnt] = 0;
    for(int i = 0;i < S.size();i++){
        if(S[i] == 'A')cnt++;
        else           cnt--;

        A[cnt] = min(A[cnt], i+1);
        B[cnt] = i+1;
    }
    int maxi = 0;
    for(int i = 0;i <= MAX_N*2;i++){
        maxi = max(maxi, B[i] - A[i]);
    }
    cout << maxi << endl;
}