source_code: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintWriter;
  import java.util.HashSet;
  import java.util.Scanner;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   *
   * @author xwchen
   */
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          Scanner in = new Scanner(inputStream);
          PrintWriter out = new PrintWriter(outputStream);
          TaskC solver = new TaskC();
          solver.solve(1, in, out);
          out.close();
      }
  
      static class TaskC {
          public void solve(int testNumber, Scanner in, PrintWriter out) {
              int n = in.nextInt();
              String s = in.next();
  
              int[] cnt = new int[26];
              int[] cnt2 = new int[26];
              int j = 0;
              int res = n;
              HashSet<Character> set = new HashSet<Character>();
              for (char c : s.toCharArray()) {
                  set.add(c);
              }
              int m = set.size();
  
              for (int i = 0; i < n; ++i) {
                  if (Character.isLowerCase(s.charAt(i))) {
                      cnt[s.charAt(i) - 'a']++;
                  } else {
                      cnt2[s.charAt(i) - 'A']++;
                  }
                  while (isOK(cnt, cnt2, m)) {
                      res = Math.min(res, i - j + 1);
                      if (Character.isLowerCase(s.charAt(j))) {
                          cnt[s.charAt(j) - 'a']--;
                      } else {
                          cnt2[s.charAt(j) - 'A']--;
                      }
                      ++j;
                  }
              }
              out.println(res);
          }
  
          boolean isOK(int[] cnt, int[] cnt2, int m) {
              int c = 0;
              for (int i = 0; i < 26; ++i) {
                  if (cnt[i] > 0) {
                      ++c;
                  }
              }
              for (int i = 0; i < 26; ++i) {
                  if (cnt2[i] > 0) {
                      ++c;
                  }
              }
              return c >= m;
          }
  
      }
  }
  
  