//最长回文字符串
//
//有a的时候报错，
int main()
{
char s[1000] = { 0 };
char m[1000] = { 0 };

scanf("%s", s);
int n = strlen(s);
int count = 0;
int max = 0;
for (int i = 0; i < n; i++)
{
    if (s[i] == s[i + 1])
    {
        int l = i;
        int r = i + 1;
        while (s[l--] == s[r++])
        {
            count = r - l;
        }
        l++;
        r--;
        if (count > max)
        {
            max = count;
            int j = 0;
            while (l <= r)
            {

                m[j++] = s[l++];
            }
        }

    }
    if (s[i] == s[i + 2])
    {
        int l = i;
        int r = i + 2;
        while (s[l--] == s[r++])
        {
            count = r - l;
        }
        l++;
        r--;
        if (count > max)
        {
            max = count;
            int j = 0;
            while (l <= r)
            {

                m[j++] = s[l++];
            }
        }
    }
    count = 0;
}

printf("%s", m);

return 0;

}