#include <bits/stdc++.h>
using namespace std;
int Substr(string str, string sub) {
  int nRet = 0, nStart = 0;
  while (-1 != (nStart = str.find(sub, nStart))) {
    nStart += sub.length();
    ++nRet;
  }
  return nRet;
}
int main() {
  string s;
  int n;
  while (cin >> n >> s) {
    map<string, int> mp;
    set<string> st;
    for (int a = 0; a < s.size() - 1; a++) {
      string y = "";
      y += s[a];
      y += s[a + 1];
      st.insert(y);
      mp[y]++;
    }
    int maxn = 0;
    for (auto &a : st)
      if (mp[a] > maxn) s = a, maxn = mp[a];
    cout << s << endl;
  }
  return 0;
}
