#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep3(i, l, n) for (int i = l; i < n; ++i)
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define out(a) cout << a << endl
#define outa(a, n) rep(_, n) cout << a[_] << " "; cout << endl
#define SZ(v) (int)v.size()
#define inf (int)(1e9+7)

vector<int> v[500001];
int main() {
    string s;
    cin >> s;
    int table[200001]; fill_n(table, 200001, -1);
    table[0] = 300000;
    v[300000].eb(0);
    rep(i, SZ(s)) {
        if (s[i] == 'A') table[i + 1] = table[i] + 1;
        else table[i + 1] = table[i] - 1;
        v[table[i + 1]].eb(i + 1);
    }
    int ans = 0;
    rep(i, 500001) {
        if (SZ(v[i]) < 2) continue;
        chmax(ans, v[i][SZ(v[i]) - 1] - v[i][0]);
    }
    out(ans);
}