import java.util.*; 

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

		String s = sc.next();
		int cnt = 0;
		int ans = 0;

		char [] a = s.toCharArray();

		for(int i=0;i<a.length;i++){
		  if(a[i]=='A' ||a[i]=='C' ||a[i]=='G'||a[i]=='T') {
				 cnt++;
		  	} else {
		//連続が終わった時の長さをansに入れる		  
				  if(ans<cnt){
					ans=cnt;
					cnt = 0;
				  }
			  }
		}
		if(ans<cnt){
			ans=cnt;
		}
		System.out.println(ans);
	  }
	}
	
/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