#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cctype>
#include <cstdlib>
#include <fstream>
#include <vector>
#include <string>
#include <sstream>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <list>
#include <bitset>
#include <numeric>
#include <algorithm>
#include <functional>
using namespace std;

#define PI 2*acos(0.0)
#define FOR(i,n) for(int i = 0;i<n;++i)
#define setbit(a,b) a|=(1<<b)
#define S1(a) scanf("%d",&a)
#define S2(a,b) scanf("%d %d",&a,&b)
#define S3(a,b,c) scanf("%d %d %d",&a,&b,&c)
#define C1(a) __builtin_popcount(a)
#define gcd(a,b) __gcd(a,b)
#define ALL(a) (a.begin(),a.end())

typedef long long LL;
typedef vector<int> vi;
const int INF = (1LL<<31)-1;

char inpt[1000005];
map< string,int > mp;

int main(){

    int n;
    while( S1(n)==1 )
    {
        mp.clear();
        scanf("%s",inpt);
        string subStr = "";
        int mx = 0,r = 0;
        //i am not using strlen because it takes time :( inpt[i] works fine.
        for(int i = 0;inpt[i];++i)
        {
            subStr += inpt[i];
            //other than making substr every time i can just erase the first character.
            // say after "abc" when d comes i will make it "abcd" and then remove a, thus it will become "bcd".
            if( i >= n )
            {
                subStr.erase(subStr.begin(),subStr.begin()+1);
            }
            //accesing map consumes log(n) time. so i will store the map value in a variable and later i will only use this variable.
            int cur = mp[ subStr ]++;
            ++cur;
            if( mx < cur )
            {
                mx = cur;
                //if we copy the result string every time it takes a bit time also :P
                //i will only kow the first index from where to print the result.
                r = i-n+1;
            }
        }
        for(int i = r;i<r+n;++i)printf("%c",inpt[i]);
        puts("");
    }
    return 0;

}