#include <bits/stdc++.h>
#define FastIO ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
using namespace std;
typedef long long ll;
int main() {
    FastIO;
    bool m;
    ll i, j, cntr, ans;
    string dns;
    cin >> dns;
    ans = 1;
    for(i = 0; i < 4; i++) {
        vector<ll> same(dns.size() * 2 + 1, -1);
        vector<ll> acc(dns.size());
        cntr = dns.size();
        for(j = 0; j < dns.size(); j++) {
            m = (dns[j] == "ACGT"[i]);
            cntr += m * 2 - 1;
            acc[j] = (j ? acc[j - 1] : 0) + m;
            if(same[cntr] == -1)
                same[cntr] = j;
            else
                ans = max(ans, min(((ll) dns.size()), (acc[j] - acc[same[cntr]]) * 2));
        }
    }
    cout << ans << '\n';
    return 0;
}