#include <stdio.h>

#include <string.h>

#define SIZE 100000 + 1

int P[SIZE * 2];

// Transform S into new string Q with special characters inserted to avoid bound checking.

void convertToNewString(const char* s, char* newString) {

int i;

newString[0] = '@';

int len = strlen(s);

for (i = 0; i < len; i++) {

newString[2 * i + 1] = '#';

newString[2 * i + 2] = s[i];

}

newString[2 * len + 1] = '#';

newString[2 * len + 2] = '$';

newString[2 * len + 3] = '\0';

}

char* longestPalindromeSubstring(const char* s, char* result) {

char Q[SIZE * 2];

int c = 0, r = 0; // current center, right limit

convertToNewString(s, Q);

int len = strlen(Q);

for (int i = 1; i < len - 1; i++) {

// find the corresponding letter in the palindrome substring

int iMirror = c - (i - c);

if (r > i) {

P[i] = (r - i) < P[iMirror] ? (r - i) : P[iMirror];

}

// expanding around center i

while (Q[i + 1 + P[i]] == Q[i - 1 - P[i]]) {

P[i]++;

}

// Update c, r in case if the palindrome centered at i expands past r

if (i + P[i] > r) {

c = i; // next center = i

r = i + P[i];

}

}

// Find the longest palindrome length in P

int maxPalindrome = 0;

int centerIndex = 0;

for (int i = 1; i < len - 1; i++) {

if (P[i] > maxPalindrome) {

maxPalindrome = P[i];

centerIndex = i;

}

}

strncpy(result, s + (centerIndex - 1 - maxPalindrome) / 2, maxPalindrome);

result[maxPalindrome] = '\0';

return result;

}

int main() {

char s[SIZE];

char result[SIZE];

fgets(s, sizeof(s), stdin);

printf("%s\n", longestPalindromeSubstring(s, result));

return 0;

}