Description：

Solution：

Code：

#include<algorithm>

#include<iostream>

#include<cstdlib>

#include<cstdio>

#include<cmath>

#include<vector>

#include<cstring>

using namespace std;

int n,m;

#define MAXN 1000010

#define MAXM 61

struct ball

{

	int col,pos;

}s[MAXM * MAXN];

int tot = 0;

bool cmp_p(ball a,ball b){return a.pos < b.pos;}

int cnt[MAXM];

int main()

{

	scanf("%d%d",&n,&m);

	int w;

	for(int i = 1;i <= m;++i)

	{

		scanf("%d",&w);

		for(int k = 1;k <= w;++k)

		{

			++tot;s[tot].col = i;

			scanf("%d",&s[tot].pos);

		}

	}

	sort(s + 1,s + 1 + tot,cmp_p);

	int l = 1,r = 0;

	int sum = 0;

	int ans = 0x3f3f3f3f;

	while(r < tot)

	{

		++r;++cnt[s[r].col];if(cnt[s[r].col] == 1)++sum;

		while(cnt[s[l].col] > 1){--cnt[s[l].col];++l;}

		if(sum == m)ans = min(ans,s[r].pos - s[l].pos);

	}

	cout << ans << endl;

	return 0;

}