#include<iostream>
#include<stdio.h>
#include<string.h>
using namespace std;
char v[53];int ch[53];
char vi[53][5],*punt=NULL,vf[53],vc[53],vp[53],*a,vt[53],chr[3];
int maxl=0,maxt=0,k,b,max2,l=0;
int n,j=0,i,ind;

main()
{
    gets(v);
    n=strlen(v);
    if(n==1){cout<<v;}
    else{



        for(i=0;i<n-1;i++)
            {   // carga inicial
               vi[i][0]=v[i] ;
               vi[i][1]=v[i+1] ;
               ch[i]=0;


            }

        for(i=0;i<n-1;i++)
        {
            punt=strstr(v,vi[i]);


            if(punt!=NULL)
                {
                    ch[punt-v]++;


                }
        }
        for(i=0;i<n-1;i++)
        {

            if(ch[i]>maxt)
                    {
                        maxt=ch[i];
                        ind=i;

                    }



        }
        strcpy(vc,vi[ind]);
        strcpy(vp,v);

        punt=strstr(v,vi[ind]);
        i=0;
 strcpy(vf,vi[ind]);
        if (maxt!=0)
        {  while(1)

        {




            b=1;

            punt=strstr(v,vf);
            chr[0]=punt[2+l];

            strcat(vf,chr);



           k=2+l;
           k++;
           l++;
           if(l==strlen(punt))break;
           max2=0;
           strcpy(vp,v);
           while(1)
               {
                    punt=strstr(vp,vf);



                    if(punt!=NULL)
                        {

                            j=strlen(punt);

                            for(i=0;i<j-1;i++)
                            {
                                vp[i]=punt[i+1];
                            }
                            vp[i]='\0';



                            max2++;
                            if (max2>=maxt)
                            {
                                maxt=max2;
                                strcpy(vc,vf);
                                b=0;

                            }




                        }
                        if(punt==NULL)break;


                }
                if(b==3) break;


        }





        }
cout<<vc;

    }}