#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Simple hash function for demonstration
unsigned long hash_string(char* s, int len) {
    unsigned long hash = 5381;
    for (int i = 0; i < len; i++) {
        hash = ((hash << 5) + hash) + s[i];
    }
    return hash;
}

char* has_duplicate_of_length(char* s, int n, int length) {
    if (length == 0) return NULL;
    
    // Simple O(n²) approach for C implementation
    for (int i = 0; i <= n - length; i++) {
        for (int j = i + 1; j <= n - length; j++) {
            if (strncmp(s + i, s + j, length) == 0) {
                char* result = (char*)malloc(length + 1);
                strncpy(result, s + i, length);
                result[length] = '\0';
                return result;
            }
        }
    }
    return NULL;
}

char* solution(char* s) {
    int n = strlen(s);
    if (n <= 1) {
        char* result = (char*)malloc(1);
        result[0] = '\0';
        return result;
    }
    
    int left = 0, right = n - 1;
    char* result = NULL;
    
    while (left <= right) {
        int mid = (left + right) / 2;
        char* duplicate = has_duplicate_of_length(s, n, mid);
        
        if (duplicate != NULL) {
            if (result) free(result);
            result = duplicate;
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }
    
    if (result == NULL) {
        result = (char*)malloc(1);
        result[0] = '\0';
    }
    return result;
}

int main() {
    char s[100001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;
    
    char* result = solution(s);
    printf("%s\n", result);
    free(result);
    return 0;
}