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 ≤ i 1 < i 2 < ... < i t ≤ | s | . The selected sequence must meet the following condition: for every j such that 1 ≤ j ≤ | s | - m + 1 , there must be at least one selected index that belongs to the segment j , j + m - 1 , i.e. there should exist a k from 1 to t , such that j ≤ i k ≤ j + m - 1 . Then we take any permutation p of the selected indices and form a new string s i p 1 s i p 2 ... s i p t . Find the lexicographically smallest string, that can be obtained using this procedure. The first line of the input contains a single integer m ( 1 ≤ m ≤ 100 000 ). The second line contains the string s consisting of lowercase English letters. It is guaranteed that this string is non-empty and its length doesn't exceed 100 000 . It is also guaranteed that the number m doesn't exceed the length of the string s . 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 ".