#include <bits/stdc++.h>
using namespace std;
string a;
int maxn = -99999999;
int weizhi = -1;
int n;
int __find__(int x) {
  int sum = 0;
  for (int i = 0; i < a.size() - 1; i++) {
    int j = i;
    if (a[j] == a[x] && a[j + 1] == a[x + 1]) sum++;
  }
  return sum;
}
int main() {
  cin >> n;
  cin >> a;
  for (int i = 0; i < a.size() - 1; i++) {
    int tmp = __find__(i);
    if (maxn < tmp) weizhi = i, maxn = tmp;
  }
  cout << a[weizhi] << a[weizhi + 1];
  return 0;
}
