#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  string s;
  cin.ignore();
  cin >> s;
  int x = 0, count = 0;
  string ans = "";
  for (int i = 0; i < (n - 1); ++i) {
    string s1 = "";
    s1 += s[i];
    s1 += s[i + 1];
    for (int j = 0; j < (n - 1); ++j) {
      string s2 = "";
      if (j != i) {
        s2 += s[j];
        s2 += s[j + 1];
      }
      if (s1 == s2) ++count;
    }
    if (x < count) {
      x = count;
      ans = s1;
    } else if (x == count) {
      if (s1 < ans) ans = s1;
    }
    count = 0;
  }
  if (ans == "") {
    char min = s[0];
    int index = 0;
    for (int i = 0; i < n; ++i) {
      if (min > s[i]) {
        min = s[i];
        index = i;
      }
    }
    if (index != (n - 1)) {
      ans += s[index];
      ans += s[index + 1];
    } else {
      ans += s[index - 1];
      ans += s[index];
    }
  }
  cout << ans << '\n';
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  solve();
}
