import java.util.*;
 
public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int max = 0;
        for(int i = 0; i < s.length(); i++){
        	int count = 0;
            for(int x = i; x < s.length(); x++){
                if(s.charAt(x) == 'A' || s.charAt(x) == 'C' || s.charAt(x) == 'G' || s.charAt(x) == 'T'){
                	count++;
                }else{
                	break;
                }
            }
            if(max < count){
            	max = count;
            }
        }
        System.out.println(max);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz