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Some people claim that a word *A* is crunchier than a word *B* if the first letter of *A* that differs from *B* is smaller ('smaller' means an earlier letter in the alphabet, eg. b is smaller than c, g is smaller than z).

You are given *N* words, each of length *K*. You can join all of them together in any order you like to form a long word. Output the crunchiest word you can form.

## Input

On the first line are integers *N* and *K* (1 ≤ *N* ≤ 20,000, 1 ≤ *K* ≤ 10).

Each of the next *N* lines contains a word of length *K* consisting only of the letters a..z.

## Output

On the first and only line, output the crunchiest word you can form.## Grading

For 30% of test cases,*N*≤ 1,000.

## Sample Input 1

3 3 ghi abc qwe

## Sample Output 1

abcghiqwe

## Explanation for Sample Output 1

There are 6 possible words you can form: ghiabcqwe, ghiqweabc, abcghiqwe, abcqweghi, qweabcghi, qweghiabc. The 4 words that start with g or q are less crunchy than the 2 that start with a. Between "abcghiqwe" and "abcqweghi", the first letter that differs is the fourth one, and g is smaller than q, hence "abcghiqwe" is crunchier.### Tags

### Subtasks and Limits

Subtask | Score | #TC | Time | Memory | Scoring |
---|---|---|---|---|---|

1 | 30 | 6 | 1s | 32MB | Minimum |

2 | 70 | 14 | 1s | 32MB | Minimum |

3 | 0 | 1 | 1s | 32MB | Minimum |

### Judge Compile Command

g++-7 ans.cpp -o crunchy -Wall -static -O2 -lm -m64 -s -w -std=gnu++17 -fmax-errors=512