#include <iostream>
#include <unordered_map>
#include <string>
#include <climits>
 
using namespace std;
 
string find_representative_string(const string& s) {
    int n = s.size();
    int max_count = 0;
    string result = "";
    unordered_map<string, int> freq;
 
    // 부분 문자열 길이를 최대 10으로 제한 (효율적인 범위로 제한)
    int max_len = min(n, 10); 
 
    // 슬라이딩 윈도우 기법으로 부분 문자열을 계산
    for (int length = 1; length <= max_len; ++length) {  // 부분 문자열의 길이
        for (int i = 0; i <= n - length; ++i) {  // 부분 문자열의 시작 인덱스
            string substring = s.substr(i, length);
            freq[substring]++;
 
            // 빈도수와 사전 순으로 조건 체크
            if (freq[substring] > max_count || 
                (freq[substring] == max_count && substring < result)) {
                max_count = freq[substring];
                result = substring;
            }
        }
    }
 
    return result;
}
 
int main() {
    string s;
    cin >> s;  // 문자열 입력
 
    // 결과 출력
    cout << find_representative_string(s) << endl;
 
    return 0;
}
 

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