Substring Order I

You are given a string of length n. If all of its distinct substrings are ordered lexicographically, what is the kth smallest of them? Input The first input line has a string of length n that consists of characters a–z. The second input line has an integer k. Output Print the kth smallest distinct substring in lexicographical order. Constraints - 1 \le n \le 10^5 - 1 \le k \le \frac{n(n+1)