479_B

#include <iostream>
#include <algorithm>
using namespace std;
int main() {
    int n;
    string s;
    cin >> n >> s;
    int max = 0;
    string ans = "";
    for (int i = 0; i < n; i += 2) {
        string tmp = s.substr(i, 2);
        int cnt = 0;
        for (int i = 0; i < n - 1; ++i) {
            string str = s.substr(i, 2);
            if (tmp == str) {
                ++cnt;
            }
        }
        if (cnt > max) {
            max = cnt;
            ans = tmp;
        }
    }
    cout << ans << endl;
}