import java.util.Scanner;

public class Main{
    public static void main(String[] args){
	Scanner scan = new Scanner(System.in);

	String t = scan.next();

	int len = 0;
	for(int i = 0; i < t.length(); i++){
	    if(t.charAt(i) == 'A' || t.charAt(i) == 'C' ||
	       t.charAt(i) == 'G' || t.charAt(i) == 'T'){
		int count = 1;
		for(int j = i+1; j < t.length(); j++){
		    if(t.charAt(j) == 'A' || t.charAt(j) == 'C' ||
		       t.charAt(j) == 'G' || t.charAt(j) == 'T'){
			count++;
		    }else{
			break;
		    }
		}
		if(len < count){
		    len = count;
		}
	    }
	}

	System.out.println(len);
    }
}

/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