#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, count = 0, max = 0;
  string str, s[105], sss, ss;
  cin >> n >> str;
  size_t npos = -1;
  ss = str;
  for (long long i = 0; i < n - 1; i++) {
    sss.push_back(str[i]);
    sss.push_back(str[i + 1]);
    while (str.find(sss) != npos) {
      str.replace(str.find(sss), 1, "*");
      count++;
    }
    s[count] = sss;
    if (count > max) max = count;
    str = ss;
    count = 0;
    sss.clear();
    continue;
  }
  cout << s[max];
}
