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

public class broken {

    public static boolean in (char[] arr, char c) {
        for (char x: arr) if (x == c) return true;
        return false;
    }

    public static void main (String... args) throws Exception {
        BufferedReader reader = new BufferedReader(new FileReader("broken.in"));
        int n = Integer.parseInt(reader.readLine());
        while (n > 0) {
            String in = reader.readLine();
            char[] c = in.toCharArray();
            int max = 0;
            for (int i = 0; i < c.length; i++) {
                int tmp = 0;
                int used_keys = 0;
                char[] keys = new char[n];
                for (int j = i; j < c.length; j++) {
                    if (in(keys, c[j])) {
                        tmp++;
                    }
                    else {
                        if (used_keys < n) {
                            keys[used_keys] = c[j];
                            used_keys++;
                            tmp++;
                        }
                        else {
                            break;
                        }
                    }
                }
                max = (max > tmp) ? max : tmp;
            }
            System.out.println(max);
            n = Integer.parseInt(reader.readLine());
        }
    }
}