• 주어진 문자열과 LCS가 특정 길이가 되는 문자열을 찾는 문제. 간단한 방법으로 O(n)에 처리 가능하다
  • 주어진 S와의 LCS의 길이가 최소가 되는 수열 찾기 참고.

"""Solution code for "BOJ 15440. Vera And LCS".

- Problem link: https://www.acmicpc.net/problem/15440
- Solution link: http://www.teferi.net/ps/problems/boj/15440

Tags: [lcs]
"""

import collections
import operator
import string


def main():
    N, K = [int(x) for x in input().split()]
    A = input()

    counter = collections.Counter(A)
    counter.update({x: 0 for x in string.ascii_lowercase})
    fewest_char, fewest_char_count = min(
        counter.items(), key=operator.itemgetter(1)
    )
    if K < fewest_char_count or K > N:
        print('WRONGANSWER')
        return

    answer = [fewest_char] * N
    a_iter = enumerate(A)
    for _ in range(K - fewest_char_count):
        i, ch = next((i, ch) for i, ch in a_iter if ch != fewest_char)
        answer[i] = ch

    print(''.join(answer))


if __name__ == '__main__':
    main()