content: import java.io.*;
  import java.math.*;
  import java.security.*;
  import java.text.*;
  import java.util.*;
  import java.util.concurrent.*;
  import java.util.regex.*;
  
  public class Solution {
  	static int validate(char c1, char c2, String s) {
  		char next = '?';
  		int counter = 0;
  		for (int i = 0; i < s.length(); i++) {
  			char current = s.charAt(i);
  			if (next == '?') {
  				if (current == c1) {
  					counter++;
  					next = c2;
  				} else if (current == c2) {
  					counter++;
  					next = c1;
  				}
  			} else {
  				if (current == c1 && current != next) { return 0; }
  				if (current == c2 && current != next) { return 0; }
  				if (current == c1 && current == next) { counter++; next = c2; }
  				if (current == c2 && current == next) { counter++; next = c1; }
  			}
  		}
  
  		return counter;
  	}
  
  	// Complete the alternate function below.
  	static int alternate(String s) {
  		if (s.length() < 2) {
  			return 0;
  		}
  
  		int max = Integer.MIN_VALUE;
  		for (int i = 0; i < 26; i++) {
  			for (int j = i + 1; j < 26; j++) {
  				max = Math.max(max, validate((char) (i + 97), (char)(j + 97), s));
  			}
  		}
  
  		return max;
  	}
  
  	public static void main(String[] args) throws IOException {
  		BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
  		BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));
  
  		int l = Integer.parseInt(bufferedReader.readLine().trim());
  
  		String s = bufferedReader.readLine();
  
  		int result = alternate(s);
  
  		bufferedWriter.write(String.valueOf(result));
  		bufferedWriter.newLine();
  
  		bufferedReader.close();
  		bufferedWriter.close();
  	}
  }
  