#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
#pragma GCC optimize("unroll-loops")
using namespace std;
long long MOD = 998244353;
double eps = 1e-12;
void solve() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  pair<char, int> p, p1;
  map<pair<char, char>, int> maap;
  int mx = 0;
  for (int i = 0; i < n - 1; i++) {
    p1 = make_pair(s[i], s[i + 1]);
    maap[p1]++;
    if (maap[p1] > mx) {
      mx = maap[p1];
      p = p1;
    }
  }
  cout << p.first << (char)p.second << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1;
  for (int it = 1; it <= t; it++) {
    solve();
  }
  return 0;
}
