#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long n;
  string s;
  cin >> n;
  cin >> s;
  vector<string> d;
  for (long long i = 1; i < n; i++) {
    string s1 = "";
    s1 += s[i - 1];
    s1 += s[i];
    d.push_back(s1);
  }
  long long m = 0;
  string ans = "";
  for (long long i = 0; i < d.size(); i++) {
    long long k = 0;
    for (long long j = 0; j < n - 1; j++) {
      if (s.substr(j, 2) == d[i]) {
        k++;
      }
    }
    if (k > m) {
      ans = d[i];
      m = k;
    }
  }
  cout << ans;
}
