Chef and LCS

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Recently, Chef learned how to solve the Longest common subsequence problem. Being a fan of Ekta Kapoor^{1}, he really likes problems which require finding something k^{th}. Please help him solve one such problem he encountered.
Chef wants to know the lexicographically k^{th} longest common subsequence of any two given strings A and B. In other words, let L be the length of LCS(A, B), S be the sorted set of all common sequences of A and B with length L, you should find S_{k}. Keep in mind that all elements of a set are distinct.
Input
The first line of input contains two integers n and k. The second line contains the string A, and the third contains the string B. Lengths of both A and B equal n.Output
Output lexicographically k^{th} longest common subsequence. If such a sequence doesn't exist, output 1.Constraints
 1 ≤ n ≤ 1000
 1 ≤ k ≤ 10^{9}
 Each character of A and B is a lowercase Latin letter.
Example
Input: 3 3 abc cba Output: c Input: 5 5 abcba bacab Output: 1 Input: 2 2 aa ab Output: 1
Explanation:
Example case 1. L = LCS(A, B) = 1. There are three common sequences with length L, S = {"a", "b", "c"}. Answer is "c".
Example case 2. L = LCS(A, B) = 3. There are four common sequences with length L, S = {"aca", "acb", "bca", "bcb"}. Answer is "1".
Example case 3. L = LCS(A, B) = 1. There is only one distinct common sequence with length L, S = {"a"}. Answer is "1".
Note: ^{1}Chef is not really a fan of Ekta Kapoor.Author:  mgch 
Editorial  http://discuss.codechef.com/problems/CHEFKLCS 
Tags  cook65, dp+lcs, dynamicprogramming, lcs, medium, mgch 
Date Added:  9122015 
Time Limit:  1 sec 
Source Limit:  50000 Bytes 
Languages:  C, CPP14, JAVA, PYTH, PYTH 3.5 
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