#include <iostream>
#include <unordered_map>
#include <string>
using namespace std;

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

    unordered_map<int, int> firstOccur;
    int balance = 0;
    int maxLen = 0;

    // ?????balance=0 ????? -1??????
    firstOccur[0] = -1;

    for (int i = 0; i < S.length(); i++) {
        if (S[i] == 'A') {
            balance++;
        } else { // S[i] == 'B'
            balance--;
        }

        if (firstOccur.find(balance) != firstOccur.end()) {
            // ?????????? balance??????????
            maxLen = max(maxLen, i - firstOccur[balance]);
        } else {
            // ??????? balance?????
            firstOccur[balance] = i;
        }
    }

    cout << maxLen << endl;
    return 0;
}