package stonehee.baekjoon;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.util.HashMap;

import java.util.Map;

public class NyanCat {

public static void main(String[] args) throws IOException {

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

int N = Integer.parseInt(br.readLine());

String str = br.readLine();

System.out.println(findMaxLength(str, N));

}

private static int findMaxLength(String str, int N) {

Map<Character, Integer> charCount = new HashMap<>();

int start = 0, maxLength = 0;

for (int end = 0; end < str.length(); end++) {

charCount.put(str.charAt(end), charCount.getOrDefault(str.charAt(end), 0) + 1);

while (charCount.size() > N) {

char startChar = str.charAt(start);

charCount.put(startChar, charCount.get(startChar) - 1);

if (charCount.get(startChar) == 0) {

charCount.remove(startChar);

}

start++;

}

maxLength = Math.max(maxLength, end - start + 1);

}

return maxLength;

}

}