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

    import java.io.*;
    import java.util.*;
    import java.math.*;
     
    public class bhaa {
     
        InputStream is;
        PrintWriter o;
        /////////////////// CODED++ BY++ ++ ++ ++ BHAVYA++ ARORA++ ++ ++ ++ FROM++ JAYPEE++ INSTITUTE++ OF++ INFORMATION++ TECHNOLOGY++ ////////////////
     
    ///////////////////////// Make it work, make it right, make it fast. Make it work, make it right, make it fast. Make it work, make it right, make it fast. Make it work, make it right, make it fast. /////////////////
     
    
        
        void solve() {
            int n=ni();
            char s[]=ns().toCharArray();
           HashMap<String,Integer>hm=new HashMap<String,Integer>();
           for(int i=0;i<n-1;i++)
           {
                String st=""+s[i]+s[i+1];
                if(hm.containsKey(st)==true)
                {
                    hm.put(st,hm.get(st)+1);
                }
                else
                {
                    hm.put(st,1);
                }
           }
           int max=0;
           String res="";
           for(String k:hm.keySet())
           {
            if(hm.get(k)>max)
            {
                max=hm.get(k);
                res=k;
            }

           }
           o.println(res);
     
        
     
            
            
     
            
     
     
        }
     
     
     
     
     
     
       
     
     
        
     
     
     
     
     
     
     
     
     
     
     
     
        
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
        //---------- I/O Template ----------
        
        public static void main(String[] args) { new bhaa().run(); }
        void run() { 
            is = System.in; 
            o = new PrintWriter(System.out);
            solve();
            o.flush();
        }
        
        byte input[] = new byte[1024];
        int len = 0, ptr = 0;
        
        int readByte() { 
            if(ptr >= len) { ptr = 0; 
                try { len = is.read(input); } 
                catch(IOException e) { throw new InputMismatchException(); } 
                if(len <= 0) { return -1; } 
            } return input[ptr++];
        }
        boolean isSpaceChar(int c) { return !( c >= 33 && c <= 126 ); }
        int skip() { 
            int b = readByte(); 
            while(b != -1 && isSpaceChar(b)) { b = readByte(); } 
            return b;
        }
        
        char nc() { return (char)skip(); }
        String ns() { 
            int b = skip(); 
            StringBuilder sb = new StringBuilder(); 
            while(!isSpaceChar(b)) { sb.appendCodePoint(b); b = readByte(); } 
            return sb.toString();
        }
        String nLine() { 
            int b = skip(); 
            StringBuilder sb = new StringBuilder(); 
            while( !(isSpaceChar(b) && b != ' ') ) { sb.appendCodePoint(b); b = readByte(); } 
            return sb.toString();
        }
        int ni() { 
            int n = 0, b = readByte(); 
            boolean minus = false; 
            while(b != -1 && !( (b >= '0' && b <= '9') || b == '-')) { b = readByte(); } 
            if(b == '-') { minus = true; b = readByte(); } 
            if(b == -1) { return -1; }  //no input 
            while(b >= '0' && b <= '9') { n = n * 10 + (b - '0'); b = readByte(); } 
            return minus ? -n : n;
        }
        long nl() { 
            long n = 0L;    int b = readByte(); 
            boolean minus = false; 
            while(b != -1 && !( (b >= '0' && b <= '9') || b == '-')) { b = readByte(); } 
            if(b == '-') { minus = true; b = readByte(); } 
            while(b >= '0' && b <= '9') { n = n * 10 + (b - '0'); b = readByte(); } 
            return minus ? -n : n;
        }
     
        double nd() { return Double.parseDouble(ns()); }
        float nf() { return Float.parseFloat(ns()); }
        int[] nia(int n) { 
            int a[] = new int[n]; 
            for(int i = 0; i < n; i++) { a[i] = ni(); } 
            return a;
        }
        long[] nla(int n) { 
            long a[] = new long[n]; 
            for(int i = 0; i < n; i++) { a[i] = nl(); } 
            return a;
        }
        int [][] nim(int n)
        {
            int mat[][]=new int[n][n];
            for(int i=0;i<n;i++)
            {
                for(int j=0;j<n;j++)
                {
                    mat[i][j]=ni();
                }
            }
            return mat;
        }
        long [][] nlm(int n)
        {
            long mat[][]=new long[n][n];
            for(int i=0;i<n;i++)
            {
                for(int j=0;j<n;j++)
                {
                    mat[i][j]=nl();
                }
            }
            return mat;
        }
        
     
     
     
     
        
        char[] ns(int n) { 
            char c[] = new char[n]; 
            int i, b = skip(); 
            for(i = 0; i < n; i++) { 
                if(isSpaceChar(b)) { break; } 
                c[i] = (char)b; b = readByte(); 
            } return i == n ? c : Arrays.copyOf(c,i);
        }
        void piarr(int arr[])
        {
            for(int i=0;i<arr.length;i++)
            {
                o.print(arr[i]+" ");
     
            }
            o.println();
        }
        void plarr(long arr[])
        {
            for(int i=0;i<arr.length;i++)
            {
                o.print(arr[i]+" ");
     
            }
            o.println();
        }
        
        void pimat(int mat[][])
        {
            for(int i=0;i<mat.length;i++)
            {
                for(int j=0;j<mat[0].length;j++)
                {
                    o.print(mat[i][j]+" ");
                }
                o.println();
            }
        }
        void plmat(long mat[][])
        {
            for(int i=0;i<mat.length;i++)
            {
                for(int j=0;j<mat[0].length;j++)
                {
                    o.print(mat[i][j]+" ");
                }
                o.println();
            }
     
        }
       static class DJset
{
    int n;
    ty1 arr[];
    class ty1
    {
        int rank;
        int par;
        ty1(int p,int r)
        {
            par=p;
            rank=r;
        }
    }
    DJset(int nu)
    {
        n=nu;
        arr=new ty1[n];
        for(int i=0;i<n;i++)
        {
            arr[i]=new ty1(i,1);
        }

    }
    int find(int x)
    {
        if(arr[x].par!=x)
        {
            arr[x].par=find(arr[x].par);
        }
        return arr[x].par;
    }
    void union(int x,int y)
    {
        x=find(x);
        y=find(y);
        if(x!=y)
        {
            if(arr[x].rank<arr[y].rank)
            {
                arr[x].par=y;
            }
            else if(arr[y].rank<arr[x].rank)
            {
                arr[y].par=x;
            }
            else
            {
                arr[x].rank++;
                arr[y].par=x;

            }
        }
    }
    public String toString()
    {
        String res="";
        for(int i=0;i<n;i++)
        {
            res+=(arr[i].par+" ");
        }
        
        return res;
    }
    public int countsets()
    {
        int res=0;
        for(int i=0;i<n;i++)
        {
            if(arr[i].par==i)
            {
                res++;
            }
        }
        return res;
    }

    

    }
    
     
     
     
     
        //////////////////////////////////// template finished //////////////////////////////////////
     
     
     
    }

