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

int main() {
    fastcin;
    static char S[200000];
    cin >> S;
    int c = 0, a = 0, t = -1;
    static int l = strlen(S);
    for(int i = 0; i < l; i++) {
        if(S[i]=='A') c++;
        else if(S[i]=='B') c--;
        if(c==0) {
            a += (i - t);
            t = i;
        }
    }
    cout << a << "\n";
    return 0;
}