import java.io.*;
import java.math.*;
import java.util.*;





public class Main {

    static MyScanner in = new MyScanner();
    static   PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
    int N;
    static int max = 200001;
    static int [] dr = {1,-1,0,0};
    static int [] dc={0,0,-1,1};
    static boolean [][] grid;
    static int [] bishopr = {-1,1,-1,1};
    static int [] bishopc = {1,1,-1,-1};
    static long H,W,cnt;
    public static void main(String args[]) throws IOException {
           //this is tring manipulatioon
           String s = in.nextLine();
           int cnt =0;
           for(int i=0;i<s.length();i++){
               for(int j=i+1;j<=s.length();j++){
                   cnt = Math.max(isValid(s.substring(i, j)),cnt);
               }
           }
           out.println(cnt);

        out.flush();
        }
       
        private static int isValid(String substring) {
            boolean notfour = false;
            int  A =0;
            int T = 0;
            int  C = 0;
            int G = 0;
            for(int i=0;i<substring.length();i++){
                    if(substring.charAt(i)=='A'){
                        A = 1;
                    }
                    else if(substring.charAt(i)=='T'){
                        T = 1;
                    }
                    else if(substring.charAt(i)=='C'){
                        C =1;
                    }
                    else if(substring.charAt(i)=='G'){
                        G = 1; 
                    }else{
                        return 0;
                    }
            }
            return substring.length();
        }

        static boolean isValid(int r, int c) {
           if(r>=0&&r<H&&c>=0&&c<W){
               return true;
           }
           return false;
       }
        private static void dfs(int r, int c) {
            for(int i=0;i<4;i++){
                int nr = bishopr[i] +r;
                int nc = bishopc[i]+c;
                if(isValid(nr, nc)&&!grid[nr][nc]){
                    cnt++;
                    grid[nr][nc] = true;
                    dfs(nr, nc);
                }
            }
        }

        static boolean isEven(long a) {
            return (a&1)==0;
        }
       static  void swap (int a, int b){

        int temmp = b;
        b=a;
        a = temmp;
        }
    

    public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
}

Measured memory_bytes: 21460