#include <bits/stdc++.h>
using namespace std;

int mi[500010];
int ma[500010];

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

    int a[200010] = {};
    for (int& x : mi) x = 1 << 29;
    for (int& x : ma) x = -(1 << 29);

    mi[200010] = 0;
    ma[200010] = 0;

    for (int i = 0; i < s.size(); i++) {
        if (i) a[i] = a[i -  1];
        if (s[i] == 'A') a[i]++;
        else a[i]--;
        mi[a[i] + 200010] = min(mi[a[i] + 200010], i + 1);
        ma[a[i] + 200010] = max(ma[a[i] + 200010], i + 1);
    }

    int ans = 0;
    for (int i = 0; i < 500010; i++) {
        if (ma[i] == 1 << 29) continue;
        ans = max(ans, ma[i] - mi[i]);
    }
    cout << ans << endl;

    return 0;
}