import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        char[] arr = sc.next().toCharArray();
        int left = -1;
        int right = -2;
        int count = 0;
        for (int i = 0; i < n; i++) {
            if (isACGT(arr[i])) {
                if (left == -1) {
                    left = i;
                }
                right = i;
                count++;
            }
        }
        System.out.println(right - Math.min(left, count - 1) + 1);
    }
    
    static boolean isACGT(char c) {
        return c == 'A' || c == 'C' || c == 'G' || c == 'T';
    }
}

class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}