CF724D Dense Subsequence

You are given a string $ s $ , consisting of lowercase English letters, and the integer $ m $ . One should choose some symbols from the given string so that any contiguous subsegment of length $ m $ has at least one selected symbol. Note that here we choose positions of symbols, not the symbols themselves. Then one uses the chosen symbols to form a new string. All symbols from the chosen position should be used, but we are allowed to rearrange them in any order. Formally, we choose a subsequence of indices $ 1

The first line of the input contains a single integer $ m $ ( $ 1

Print the single line containing the lexicographically smallest string, that can be obtained using the procedure described above.

In the first sample, one can choose the subsequence $ {3} $ and form a string "a". In the second sample, one can choose the subsequence $ {1,2,4} $ (symbols on this positions are 'a', 'b' and 'a') and rearrange the chosen symbols to form a string "aab".