class Solution {
    fun palindrome(s: String, l: Int, r: Int) : Boolean {
        var u = l
        var v = r-1
        while (u < v)
            if (s[u++] != s[v--])
                return false 
        return true }
    fun longestPalindrome(s: String): String {
        val n = s.length
        for (k in n downTo 2) 
            for (l in 0 until n-k+1) 
                if (palindrome(s,l,l+k))
                    return s.substring(l,l+k)
        return s.substring(0,1) } }
 
fun main(args: Array<String>) {
    print(Solution().longestPalindrome(readLine()!!)) }

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