#include "bits/stdc++.h"
#define fastcin {\
    cin.tie(0);\
    ios::sync_with_stdio(false);\
}
using namespace std;

int main() {
    fastcin;
    string S;
    cin >> S;
    int l = S.length(), c = 0, ans = 0;
    map<int, int> m;
    m[0] = 0;
    for(int i = 0; i < l; i++) {
        if(S[i]=='A') c++;
        else if(S[i]=='B') c--;
        if(m.find(c)==m.end()) m[c] = i+1;
        else {
            if(ans < (i+1 - m[c])) ans = i+1 - m[c];
        }
    }
    cout << ans << "\n";
    return 0;
}