#include <bits/stdc++.h>
using namespace std;
bool compare(pair<string, int> a, pair<string, int> b) {
  return a.first > b.first;
}
int main() {
  int n;
  string s;
  cin >> n >> s;
  map<string, int> m;
  int l = s.length();
  for (int i = 0; i < l - 1; i++) {
    stringstream ss;
    ss << s[i] << s[i + 1];
    string t = ss.str();
    if (m.find(t) != m.end())
      m[t]++;
    else
      m.insert(make_pair(t, 1));
  }
  int max1 = -1;
  string ans = "";
  for (pair<string, int> p : m) {
    if (p.second > max1) {
      max1 = p.second;
      ans = p.first;
    }
  }
  cout << ans;
  return 0;
}
