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

int main()
{
    string a;
    cin>>a;
    int n=a.size();
    int ans=0;
    for (char most:{'A','C','T','G'}){
        int mustother=0;
        int charnow=0;
        for (int i=0; i<n;i++){
            if (a[i]==most) charnow++;
            else mustother++;
            if (mustother>charnow){
                charnow--;
            }
            if (i+1<=2*charnow){
                ans=max(i+1,ans);
            }
            else{
                ans=max(ans, 2*charnow);
            }
        }

    }
    cout<<ans;
}