15
MIRZOYANOVECLOX
 23
AABBBAAACCCCCAAADDDDDDD
 2
QA
 11
GGRRAATTZZZ
 6
AZAZAZ
 10
SQSQSQSQTG
 3
KEK
 100
URXCAIZFIBNJTPCZHBQIBCILLPXZCFGMKKZMNPLCYGAVJVIBMCZEBSJWPSCPQDYCTTKPOKIJRSKIZPDGCHVOUTMPNECYORSFZFNC
 3
LOL
 2
WW
 8
PUTINVOR
 26
QWERTYUIOPASDFGHJKLZXCVBNM
 5
AZAZA
 9
EGORLETOV
 7
KADUROV
 9
MIKEPIDOR
 9
AAAAAAAAA
 100
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 5
FUFEL
 50
NYQAHBYYOXLTRYQDMVENEMAQNBAKGLGQOLXNAIFNQTOCLNNQIA

MI
 DD
 QA
 ZZ
 AZ
 SQ
 KE
 IB
 LO
 WW
 PU
 QW
 AZ
 EG
 KA
 MI
 AA
 AA
 FU
 YQ

#	✪ H4WK3yE乡
#	Mayank Chaudhary
#	ABES EC , Ghaziabad
# ///==========Libraries, Constants and Functions=============///
import sys
from bisect import bisect_left,bisect_right
from collections import deque,Counter
from math import gcd,sqrt,factorial,ceil,log
from itertools import permutations
inf = float("inf")
mod = 1000000007
mini=1000000007

def fact(n):                                                             # <------  To calculate factorial of n under modulo m
    if n==0:
        return 1
    p=1;d=(10**9)+7
    for i in range(1,n+1):
        p=p*i
        p=p%d
    return p
def ncr(n,r):                                                          # < ------ To calculate nCr mod p value using Fermat Little under modulo m
    d=10**9+7
    num=fact(n)
    den=(fact(r)*fact(n-r))%d
    den=pow(den,d-2,d)
    return (num*den)%d
def sieve(n):                                                          # <----- sieve of eratosthenes for prime no.
    prime=[True for i in range(n+1)]
    lst=[0]*(n+1)
    p=2
    while p*p<=n:
        if prime[p]:
            lst[p]=p
            for i in range(p*p,n+1,p):
                if prime[i]==True:
                    prime[i]=False
                    lst[i]=p
        p=p+1
    for i in range(1,n+1):
        if lst[i]==0:
            lst[i]=i
    mylist=[0]*(n+1)
    for i in range(2,n+1):
        mylist[lst[i]]=mylist[lst[i]]+1
    return mylist
        
def binary(number):                                             #  <----- calculate the no. of 1's in binary representation of number
    result=0
    while number:
        result=result+1
        number=number&(number-1)
    return result

def calculate_factors(n):                                #<---- most efficient method to calculate no. of factors of number             
    hh = [1] * (n + 1); 
    p = 2; 
    while((p * p) < n): 
        if (hh[p] == 1): 
            for i in range((p * 2), n, p): 
                hh[i] = 0; 
        p += 1;  
    total = 1; 
    for p in range(2, n + 1): 
        if (hh[p] == 1):  
            count = 0; 
            if (n % p == 0): 
                while (n % p == 0): 
                    n = int(n / p); 
                    count += 1; 
                total *= (count + 1); 
    return total;

def prime_factors(n):                                         #<------------ to find prime factors of a no.
    i = 2
    factors = set()
    while i * i <= n:
        if n % i:
            i += 1
        else:
            n //= i
            factors.add(i)
    if n > 1:
        factors.add(n)
    if len(factors)==2:
        return True
    else:
        return False

def isPrime(n):                                                      #<-----------check whether a no. is prime or not
    if n==2 or n==3: return True
    if n%2==0 or n<2: return False
    for i in range(3,int(n**0.5)+1,2):   # only odd numbers
        if n%i==0:
            return False    

    return True
def get_array(): return list(map(int, sys.stdin.readline().strip().split()))
def get_ints(): return map(int, sys.stdin.readline().strip().split())
def input(): return sys.stdin.readline().strip()
# ///==========MAIN=============///
n=int(input())
string=input();max_count=0
for i in range(n-1):
    mystr=string[i:i+2]
    curr=0
    for j in range(n-1):
        if string[j:j+2]==mystr:
            curr=curr+1
    if curr>max_count:
        max_count=curr
        fav=mystr
print(fav)
