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

int main() {
    string s;
    cin >> s;
    map<int,int> mp;
    int ans = 0, cnt = 0;
    mp[0] = -1;
    REP(i,s.size()){
        cnt += s[i]=='A'?1:-1;
        if(!mp.count(cnt))mp[cnt] = i;
        ans = max(ans,i-mp[cnt]);
    }
    cout << ans << endl;
    return 0;
}