#include <stdio.h>
#define SIZE 100000

int main(void)
{

    char s[SIZE] = {0};
    gets(s);
    char subs[SIZE] = "-1\0";
    int cb = 0, ce = 0;
    int ab = 0, ae = 0;

    for (int i = 0; i < SIZE && s[i]; ++i){
        cb = i;
        ce = i;
        for (int j = i+1; j < SIZE && s[j]; ++j)
        {
            if (s[i] == s[j]){
                for (int k = 0; k < (j-i)/2; ++k){
                    if ( s[i+k] != s[j-k]){
                        ce = j;
                    }
                }
            }
            else{
                ce = j;
            }
        }
        if ( (ae - ab) < ( ce - cb))
        {
            ae = ce;
            ab = cb;
        }
    }
    if ((ae-ab) > 1){
        for (int i = ab ; i <= ae; ++i)
            subs[i-ab] = s[i];
        subs[ae-ab+1] = 0;
    }
    printf("%s", subs);

    fflush(stdin);
    getc(stdin);
    return 0;
}