15
MIRZOYANOVECLOX
 23
AABBBAAACCCCCAAADDDDDDD
 2
QA
 11
GGRRAATTZZZ
 6
AZAZAZ
 10
SQSQSQSQTG
 3
KEK
 100
URXCAIZFIBNJTPCZHBQIBCILLPXZCFGMKKZMNPLCYGAVJVIBMCZEBSJWPSCPQDYCTTKPOKIJRSKIZPDGCHVOUTMPNECYORSFZFNC
 3
LOL
 2
WW
 8
PUTINVOR
 26
QWERTYUIOPASDFGHJKLZXCVBNM
 5
AZAZA
 9
EGORLETOV
 7
KADUROV
 9
MIKEPIDOR
 9
AAAAAAAAA
 100
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 5
FUFEL
 50
NYQAHBYYOXLTRYQDMVENEMAQNBAKGLGQOLXNAIFNQTOCLNNQIA

MI
 DD
 QA
 ZZ
 AZ
 SQ
 KE
 IB
 LO
 WW
 PU
 QW
 AZ
 EG
 KA
 MI
 AA
 AA
 FU
 YQ

import java.util.*;
import java.util.function.*;
import java.io.*;
import java.math.*;
import java.awt.geom.*;
public class Main {
  public static final PrintWriter outWriter = new PrintWriter(System.out);
  public static final int _BUFFER_SIZE = 1 << 16;
  public static final DataInputStream _din = new DataInputStream(System.in);
  public static final byte[] _buffer = new byte[_BUFFER_SIZE];
  public static int _bufferPointer = 0, _bytesRead = 0;
  public static class Multiset<E> {
    HashMap<E,Integer> tree = new HashMap<E,Integer>();
    Integer add(E obj) {
      return tree.merge(obj,1,Integer::sum);
    }
    Integer remove(E obj) {
      return tree.merge(obj,-1,(a,b)->a+b == 0 ? null : a+b);
    }
    Integer get(E obj) {
      return tree.getOrDefault(obj, 0);
    }
    public String toString() { return tree.toString(); }
  }
  public static void main() {
    int n = nextInt();
    String s = next();
    Multiset<String> set = new Multiset<String>();
    for (int i = 0; i < n-1; i++) {
      set.add(s.substring(i,i+2));
    }
    String maxS = "";
    int max = 0;
    for (Map.Entry<String, Integer> e : set.tree.entrySet()) {
      if (e.getValue() > max) {
        max = e.getValue();
        maxS = e.getKey();
      }
    }
    outWriter.println(maxS);
  }
  public static void main(String[] args) { main(); outWriter.flush(); }
  public static void _fillBuffer() {
    try {
      _bytesRead = _din.read(_buffer, _bufferPointer = 0, _buffer.length);
      if (_bytesRead == -1)
        _buffer[0] = -1;
    } catch (IOException e) {
      throw new UncheckedIOException(e);
    }
  }
  public static byte _read() {
    if (_bufferPointer == _bytesRead)
      _fillBuffer();
    return _buffer[_bufferPointer++];
  }
  public static String next() {
    byte[] strBuf = new byte[64];
    int cnt = 0, c;
    while ((c = _read()) != -1) {
      if (Character.isWhitespace(c)) {
        if (cnt == 0) {
          continue;
        } else {
          break;
        }
      }
      if (strBuf.length == cnt) {
        strBuf = Arrays.copyOf(strBuf, strBuf.length*2);
      }
      strBuf[cnt++] = (byte) c;
    }
    return new String(strBuf, 0, cnt);
  }
  public static String nextLine() {
    byte[] strBuf = new byte[64];
    int cnt = 0, c;
    while ((c = _read()) != -1) {
      if (c == '\n') {
        if (cnt == 0) {
          continue;
        } else {
          break;
        }
      }
      if (strBuf.length == cnt) {
        strBuf = Arrays.copyOf(strBuf, strBuf.length*2);
      }
      strBuf[cnt++] = (byte) c;
    }
    return new String(strBuf, 0, cnt);
  }
  public static int nextInt() {
    int ret = 0;
    byte c = _read();
    while (c <= ' ')
      c = _read();
    boolean neg = (c == '-');
    if (neg)
      c = _read();
    do {
      ret = ret * 10 + c - '0';
    } while ((c = _read()) >= '0' && c <= '9');
    if (neg)
      return -ret;
    return ret;
  }
  public static long nextLong() {
    long ret = 0;
    byte c = _read();
    while (c <= ' ')
      c = _read();
    boolean neg = (c == '-');
    if (neg)
      c = _read();
    do {
      ret = ret * 10 + c - '0';
    } while ((c = _read()) >= '0' && c <= '9');
    if (neg)
      return -ret;
    return ret;
  }
  public static double nextDouble() {
    double ret = 0, div = 1;
    byte c = _read();
    while (c <= ' ')
      c = _read();
    boolean neg = (c == '-');
    if (neg)
      c = _read();
    do {
      ret = ret * 10 + c - '0';
    }
    while ((c = _read()) >= '0' && c <= '9');
    if (c == '.') {
      while ((c = _read()) >= '0' && c <= '9') {
        ret += (c - '0') / (div *= 10);
      }
    }
    if (neg)
      return -ret;
    return ret;
  }
}
