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

char* solution(char* num) {
    // Count frequency of each digit
    int count[10] = {0};
    int len = strlen(num);
    
    for (int i = 0; i < len; i++) {
        count[num[i] - '0']++;
    }
    
    // Build the first half of palindrome (largest digits first)
    char* result = malloc(len + 1);
    int pos = 0;
    char middle = '\0';
    
    // Process digits from 9 to 0 (largest to smallest)
    for (int digit = 9; digit >= 0; digit--) {
        // Add pairs to first half
        int pairs = count[digit] / 2;
        for (int i = 0; i < pairs; i++) {
            result[pos++] = '0' + digit;
        }
        
        // If we haven't chosen a middle digit and this digit has odd count
        if (middle == '\0' && count[digit] % 2 == 1) {
            middle = '0' + digit;
        }
    }
    
    // Handle edge case: all zeros
    if (pos == 0 && middle == '0') {
        strcpy(result, "0");
        return result;
    }
    
    // Handle edge case: result would have leading zeros
    if (pos > 0 && result[0] == '0') {
        if (middle == '\0') {
            strcpy(result, "0");
        } else {
            result[0] = middle;
            result[1] = '\0';
        }
        return result;
    }
    
    // Add middle if exists
    if (middle != '\0') {
        result[pos++] = middle;
    }
    
    // Add reversed first half
    int firstHalfEnd = (middle != '\0') ? pos - 2 : pos - 1;
    for (int i = firstHalfEnd; i >= 0; i--) {
        if (result[i] != middle || i != pos - 1) {
            result[pos++] = result[i];
        }
    }
    
    result[pos] = '\0';
    
    if (strlen(result) == 0) {
        strcpy(result, "0");
    }
    
    return result;
}

int main() {
    char num[1000];
    fgets(num, sizeof(num), stdin);
    num[strcspn(num, "\n")] = 0;
    
    char* result = solution(num);
    printf("%s\n", result);
    free(result);
    return 0;
}