def sub_palindrome(string):
    max_length = 1
    start = 0
    for i in range(1, len(string)):
        low = i - 1
        high = i
        while low >= 0 and high < len(string) and string[low] == string[high]:
            if high - low + 1 > max_length:
                start = low
                max_length = high - low + 1
            low -= 1
            high += 1
        low = i - 1
        high = i + 1
        while low >= 0 and high < len(string) and string[low] == string[high]:
            if high - low + 1 > max_length:
                start = low
                max_length = high - low + 1
            low -= 1
            high += 1

    return string[start:start + max_length]


string = input()
print(sub_palindrome(string))