source_code: import java.io.BufferedWriter;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.io.OutputStreamWriter;
  import java.io.PrintWriter;
  import java.io.Writer;
  import java.util.Arrays;
  import java.util.InputMismatchException;
  
  
  
  
  public class TestClass11 {
  
  	/**
  	 * @param args
  	 */
  	public static void main(String[] args) {
  		// TODO Auto-generated method stub
  		InputReader in=new InputReader(System.in);
  		OutputWriter out=new OutputWriter(System.out);
  		int n=in.readInt();
  		String s=in.readString();
  		int low[]=new int[26];
  		int upper[]=new int[26];
  		boolean islow[]=new boolean[26];
  		boolean isupper[]=new boolean[26];
  		Arrays.fill(low,Integer.MAX_VALUE);
  		Arrays.fill(upper, Integer.MAX_VALUE);
  		
  		int ans[]=new int[n];
  		int finalsans=Integer.MAX_VALUE;
  		for(int i=0;i<n;i++){
  			int c=s.charAt(i);
  			if(c>='a'&&c<='z'){
  				islow[c-'a']=true;
  			}
  			else{
  				isupper[c-'A']=true;
  			}
  			
  		}
  		
  		
  		for(int i=n-1;i>=0;i--){
  			int c=s.charAt(i);
  			
  			if(c>='a'&&c<='z'){
  				low[c-'a']=i;
  			}
  			else{
  				upper[c-'A']=i;
  			}
  			
  			
  			for(int j=0;j<26;j++){
  				if(islow[j]==true){
  					ans[i]=Math.max(ans[i], low[j]);
  				}
  			}
            for(int j=0;j<26;j++){
          	  if(isupper[j]==true){
          		  ans[i]=Math.max(ans[i], upper[j]);
  				}
  			}
            
            finalsans=Math.min(ans[i]-i+1, finalsans);
  		}
  		
  
  		System.out.println(finalsans);
  		
  		
  	}
  
  }
  
  class InputReader {
  	 
  	private InputStream stream;
  	private byte[] buf = new byte[1024];
  	private int curChar;
  	private int numChars;
  	private SpaceCharFilter filter;
   
  	public InputReader(InputStream stream) {
  		this.stream = stream;
  	}
   
  	public int read() {
  		if (numChars == -1)
  			throw new InputMismatchException();
  		if (curChar >= numChars) {
  			curChar = 0;
  			try {
  				numChars = stream.read(buf);
  			} catch (IOException e) {
  				throw new InputMismatchException();
  			}
  			if (numChars <= 0)
  				return -1;
  		}
  		return buf[curChar++];
  	}
   
  	public int readInt() {
  		int c = read();
  		while (isSpaceChar(c))
  			c = read();
  		int sgn = 1;
  		if (c == '-') {
  			sgn = -1;
  			c = read();
  		}
  		int res = 0;
  		do {
  			if (c < '0' || c > '9')
  				throw new InputMismatchException();
  			res *= 10;
  			res += c - '0';
  			c = read();
  		} while (!isSpaceChar(c));
  		return res * sgn;
  	}
   
  	public String readString() {
  		int c = read();
  		while (isSpaceChar(c))
  			c = read();
  		StringBuilder res = new StringBuilder();
  		do {
  			res.appendCodePoint(c);
  			c = read();
  		} while (!isSpaceChar(c));
  		return res.toString();
  	}
   
  	public boolean isSpaceChar(int c) {
  		if (filter != null)
  			return filter.isSpaceChar(c);
  		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
  	}
   
  	public String next() {
  		return readString();
  	}
   
  	public interface SpaceCharFilter {
  		public boolean isSpaceChar(int ch);
  	}
  }
   
  class OutputWriter {
  	private final PrintWriter writer;
   
  	public OutputWriter(OutputStream outputStream) {
  		writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
  	}
   
  	public OutputWriter(Writer writer) {
  		this.writer = new PrintWriter(writer);
  	}
   
  	public void print(Object...objects) {
  		for (int i = 0; i < objects.length; i++) {
  			if (i != 0)
  				writer.print(' ');
  			writer.print(objects[i]);
  		}
  	}
   
  	public void printLine(Object...objects) {
  		print(objects);
  		writer.println();
  	}
   
  	public void close() {
  		writer.close();
  	}
   
  	public void flush() {
  		writer.flush();
  	}
   
  	}
   
  class IOUtils {
   
  	public static int[] readIntArray(InputReader in, int size) {
  		int[] array = new int[size];
  		for (int i = 0; i < size; i++)
  			array[i] = in.readInt();
  		return array;
  	}
   
  	}
  