import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashSet;
class Main{
static StringBuilder s;
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int n = Integer.parseInt(br.readLine());
s = new StringBuilder(br.readLine());
HashSet<String> set = new HashSet<>();
String s2="";
int a[] = new int[s.length()-n];
int max = 0;
int start = 0;
int end = 0;
while(n<=s.length()){
for (int i = 0; i <= s.length()-n; i++) {
s2 = s.substring(i, i+n);
if(set.add(s2)){
int num=getNum(s,s2);
if(num > max || (max==num &&(end - start)<(n))){
start = i;
end = i+n;
max = num;
}
}
}
n++;
}
System.out.println(s.substring(start, end));
}
private static int getNum(StringBuilder s2, String s22) {
int num = 0;
int index = s2.indexOf(s22, 0);
while(index!=-1){
num++;
index = s2.indexOf(s22, index+1);
}
return num;
}
}
