#include <bits/stdc++.h>
using namespace std;
char s[105];
int cnt[105];
int main() {
  int n;
  while (cin >> n) {
    memset(cnt, 0, sizeof(cnt));
    for (int i = 0; i < n; ++i) cin >> s[i];
    for (int i = 0; i < n - 1; ++i) {
      for (int j = 0; j < n - 1; ++j)
        if (s[i] == s[j] && s[i + 1] == s[j + 1]) ++cnt[i];
    }
    int max = -1;
    string ans;
    for (int i = 0; i < n; ++i) {
      if (cnt[i] > max) {
        max = cnt[i];
        ans = "";
        ans += s[i];
        ans += s[i + 1];
      }
    }
    cout << ans << endl;
  }
}
