#include<iostream>
#include<cstring>
#include<ctype.h>
using namespace std;
int a[680];
string s;
int main(){
    ios::sync_with_stdio(false);
    memset(a, 0, sizeof(a));
    getline(cin, s);
    int len = s.size() - 1, Max = -1000000000;
    for(int i = 0; i < len; i++){
        if(isalpha(s[i]) && isalpha(s[i+1])){
            a[26*(tolower(s[i])-97)+(tolower(s[i+1])-96)]++;
        }
    }
    for(int i = 1; i <= 676; i++){
        if(a[i] > Max)Max = a[i];
    }
    for(int i = 1; i <= 676; i++){
        if(a[i] == Max){
            if(i % 26)cout << char(i/26+97) << char(i%26+96) << ' ';
            else cout << char(i/26+96) << "z ";
        }
    }
    return 0;
}