// UUID: 0abf55a7-dd3a-4bfa-bec5-c8af8f985884
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define F first
#define S second
#define pii pair<int, int>
#define pb push_back
#define srt(x) x.begin(),x.end()
const int INF = 1e9 + 7;

int ans = 1;
string s;

void calc(char c)
{
    int n = s.size();
    vector<int>a(n), pref(n);
    map<int, int>m;
    for(int i = 1; i < n; i++) 
    {
        a[i] = (s[i] == c ? 1 : -1);
        pref[i] = pref[i-1] + a[i];
        if(pref[i] >= 0)
        {
            ans = max(ans, i);
            continue;
        }
        if(m[pref[i]] > 0) ans = max(ans, i - m[pref[i]]);
        else m[pref[i]] = i;
    }
}

signed main() {
    ios::sync_with_stdio(false);cin.tie(nullptr);
    cin >> s;
    s = ' ' + s;
    calc('A');
    calc('C');
    calc('G');
    calc('T');
    cout << ans;
}