#include <bits/stdc++.h>
using namespace std;
const int INF = 2147380000;
const long long inf = 1e18;
const int maxn = 200;
map<string, int> ms;
int n;
string ans;
int Max;
char s[maxn];
int main() {
  while (scanf("%d", &n) != EOF) {
    scanf(" %s", s);
    ms.clear();
    Max = -1;
    for (int i = 0; i < n - 1; i++) {
      char S[5];
      S[0] = s[i];
      S[1] = s[i + 1];
      S[2] = '\0';
      string ss = (string)S;
      if (ms.count(ss)) {
        ms[ss]++;
      } else {
        ms[ss] = 1;
      }
    }
    for (int i = 0; i < n - 1; i++) {
      char S[5];
      S[0] = s[i];
      S[1] = s[i + 1];
      S[2] = '\0';
      string ss = (string)S;
      if (ms[ss] > Max) {
        Max = ms[ss];
        ans = ss;
      }
    }
    cout << ans << endl;
  }
  return 0;
}
