#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define llong long long

int main() {
    string s;
    cin >> s;

    bool is_a = false;
    int lower = 0;
    int upper = s.length() / 2;
    while(upper > lower) {
        int middle = (upper + lower + 1) / 2;
        int alen = 0;
        int blen = 0;
        bool flg = false;
        rep(i, middle*2) {
            if(s[i] == 'A') alen++;
            else blen++;
        }
        flg |= alen == blen;
        for(int i=1; i<s.length() - middle * 2 + 1; i++) {
            if(flg) break;
            if(s[i-1] == 'A') alen--;
            else blen--;
            if(s[i+middle*2-1] == 'A') alen++;
            else blen++;
            flg |= alen == blen;
        }
        if(flg) lower = middle;
        else upper = middle-1;
    }

    cout << upper * 2 << "\n";
}