This problem's actual name, "Lexicographically Largest Palindromic Subsequence" is too long to fit into the page headline.

You are given string s consisting of lowercase English letters only. Find its lexicographically largest palindromic subsequence.

We'll call a non-empty string s[p₁p₂... p_k] = s_p1s_p2... s_pk (1  ≤  p₁ < p₂ < ... < p_k  ≤  |s|) a subsequence of string s = s₁s₂... s_|s|, where |s| is the length of string s. For example, strings "abcb", "b" and "abacaba" are subsequences of string "abacaba".

String x = x₁x₂... x_|x| is lexicographically larger than string y = y₁y₂... y_|y| if either |x| > |y| and x₁ = y₁, x₂ = y₂, ..., x_|y| = y_|y|, or there exists such number r (r < |x|, r < |y|) that x₁ = y₁, x₂ = y₂, ..., x_r = y_r and x_{r  +  1} > y_{r  +  1}. Characters in the strings are compared according to their ASCII codes. For example, string "ranger" is lexicographically larger than string "racecar" and string "poster" is lexicographically larger than string "post".

String s = s₁s₂... s_|s| is a palindrome if it matches string rev(s) = s_|s|s_|s| - 1... s₁. In other words, a string is a palindrome if it reads the same way from left to right and from right to left. For example, palindromic strings are "racecar", "refer" and "z".