source_code: import java.util.*;
  import java.io.*;
  import java.math.*;
  
  public class Main {
  
  	BufferedReader in;
  	StringTokenizer str = null;
  	PrintWriter out;
  	
  	private String next() throws Exception{
  		while (str == null || !str.hasMoreElements())
  			str = new StringTokenizer(in.readLine());
  		return str.nextToken();
  	}
  	
  	private int nextInt() throws Exception{
  		return Integer.parseInt(next());
  	}
  	
  	private long nextLong() throws Exception{
  		return Long.parseLong(next());
  	}
  	
  	private double nextDouble() throws Exception{
  		return Double.parseDouble(next());
  	}
  
  	final int oo = Integer.MAX_VALUE;
  	
  	int [][]s;
  	int n, ALL;
  	public void run() throws Exception{
  		in =  new BufferedReader(new InputStreamReader(System.in));
  		out = new PrintWriter(System.out);
  		n = nextInt();
  		char []a = next().toCharArray();
  		s = new int[n][52];
  		boolean []set = new boolean[52];
  
  		for(int i = 0; i < n; ++i) {
  			int pos = get(a[i]);
  			if (!set[pos]) {
  				++ALL;
  				set[pos] = true;
  			}
  			for(int j = 0; j < 52; ++j) {
  				if (i > 0) {
  					s[i][j] += s[i-1][j];	
  				}
  				if (j == pos) {
  					s[i][j]++;
  				}
  			}
  		}
  
  		int ret = oo;
  		for(int i = 0; i < n; ++i) {
  			ret = Math.min(ret, get(i));
  		}
  
  		out.println(ret);
  		out.close();
  	}
  
  	private int get(int i) {
  		int lo = i - 1, hi = n;
  		while(hi - lo > 1) {
  			int m = lo + (hi - lo) / 2;
  			int c = 0;
  			for(int j = 0; j < 52; ++j) {
  				if (sum(j, i, m) > 0) {
  					++c;
  				}
  			}
  			if (c < ALL) {
  				lo = m;
  			} else {
  				hi = m;
  			}
  		}
  
  		if (hi != n) {
  			return hi - i + 1;
  		}
  		return oo;
  	}
  
  	private int sum(int pos, int l, int r) {
  		int ret = s[r][pos];
  		if (l > 0) ret -= s[l - 1][pos];
  		return ret;
  	}
  
  	private int get(char x) {
  		if ('a' <= x && x <= 'z') return (int)(x - 'a');
  		return (int)(x - 'A' + 26);
  	}
  
  	public static void main(String[] args) throws Exception{
  		new Main().run();
  	}
  }
  