// Author: Dennis Waldherr
import java.util.*;
import java.io.*;

public class broken {
	public static void main(String... args) throws Exception {
		BufferedReader in = new BufferedReader(new FileReader("broken.in"));
		String line;
		while ((line = in.readLine()) != null) {
			int m = Integer.parseInt(line);
			if (m == 0) {
				break;
			}
			line = in.readLine();
			int start = 0, end = 0;
			int[] count = new int[256];
			int diff = 0;
			int best = 0;
			while (end < line.length()) {
				if (diff < m || count[line.charAt(end)] > 0) {
					if (count[line.charAt(end)] == 0) {
						diff++;
					}
					count[line.charAt(end)]++;
					end++;
					if (end - start > best) {
						best = end - start;
					}
				} else {
					count[line.charAt(start)]--;
					if (count[line.charAt(start)] == 0) {
						diff--;
					}
					start++;
				}
			}
			System.out.println(best);
		}
		in.close();
	}
}