import java.util.Scanner;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   String str = sc.next();
   int count = 0;
   int countmem = 0;
 for (int i = 0; i < str.length(); i++) {
	if(str.charAt(i) == 'A' || str.charAt(i) == 'T' ||
       str.charAt(i) == 'C' || str.charAt(i) == 'G'){
       count++;
    }else{
      if(countmem < count){
      countmem = count;
      }
      count = 0;
    }
   if(i == str.length()-1 && countmem < count){
      countmem = count;      
   }
  }
    System.out.print(countmem);
 }
}
/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