Groundhog Looking Dowdy
时间限制：C/C++/Rust/Pascal 1秒，其他语言2秒
空间限制：C/C++/Rust/Pascal 256 M，其他语言512 M
64bit IO Format: %lld

题目描述

Groundhog finds that Apple seems to be less intimate than before.

He is very distressed for this.After pondering for a long time, Groundhog finds that he looks too dowdy.So, Groundhog decided to improve a little.

Because Groundhog is lazy, he only lists the clothes that can be worn for the next  days. On the  day, the  clothes have a dowdiness .On each day,he will choose one of the clothes and wear it.

And Groundhog should choose  days from  days to go out with Apple.

Groundhog wants to know the minimum difference between the maximum dowdiness and the minimum dowdiness in  days when Groundhog's choice is optimal.

Simplified: You should choose n clothes for each day and then choose m clothes from those n clothes, and the problem is to calculate the minimum difference between the maximum dowdiness and the minimum dowdiness.

输入描述:

The first line contains two integers  and .
Then  lines follow,each line contains a integer k_i,represents the number of the clothes that can be worn on  day.Then k_i integers  follow.

输出描述:

Print in one line the minimum difference.
示例1

输入

复制 
4 3
1 3
2 8 6
1 2
3 1 7 5

输出

复制 
2

说明

Apple will pay attention to Groundhog's clothes on day 1, 3, and 4 ,Groundhog will wear clothes with dowdiness of 3, 2, and 1 on day 1, 3, and 4,and the difference is 2.

备注:

，，the sum of clothes .