def smallest_palindrome(s):
    # Count the frequency of each character
    freq = {}
    for char in s:
        freq[char] = freq.get(char, 0) + 1
 
    # Identify characters with odd frequency
    odd_freq_chars = [char for char, count in freq.items() if count % 2 != 0]
 
    # Construct the palindrome
    palindrome = ''
    for char, count in freq.items():
        if count % 2 == 0:
            palindrome += char * (count // 2)
 
    # Sort odd frequency characters
    odd_freq_chars.sort()
 
    # Insert characters with odd frequency at the beginning and end of the palindrome
    for char in odd_freq_chars:
        palindrome = char * (freq[char] // 2) + palindrome + char * (freq[char] // 2)
 
    return palindrome
 
# Input
s = input()
 
# Output
print(smallest_palindrome(s))
 

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