killjee has got a string s and he asks you to find a palindromic sub-sequence of that string for him. If there exist multiple such string he wants the one which is laxicographically smallest.

INPUT FORMAT

Only line of input contains a string s.

OUTPUT FORMAT

Print a palindromic string which is sub-sequence of string s. If multiple answers exist output the one which is lexicographically smallest.

Print 1 if no such subsequence is present in the string.

INPUT CONSTRAINTS

$1 \le |s| \le 10^6$