import java.util.*;

/**
 * Main
 */
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		char[] c = sc.next().toCharArray();
		int max = 0;
		int count = 0;

		for (int i = 0; i < c.length; i++) {
			if (c[i] == 'A' || c[i] == 'C' || c[i] == 'G' || c[i] == 'T') {
				count += 1;
			} else {
				max = Math.max(max, count);
				count = 0;
			}
		}
		max = Math.max(max, count);
		sc.close();

		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