#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  string str;
  cin >> n;
  cin >> str;
  map<string, int> freq;
  for (int i = 0; i < (int)str.size() - 1; ++i) {
    string temp;
    temp.push_back(str[i]);
    temp.push_back(str[i + 1]);
    if (freq.find(temp) != freq.end())
      ++freq[temp];
    else
      freq.insert(pair<string, int>(temp, 1));
  }
  map<string, int>::iterator idx = freq.begin();
  for (map<string, int>::iterator it = freq.begin(); it != freq.end(); ++it)
    if (it->second > idx->second) idx = it;
  cout << idx->first << "\n";
  return 0;
}
