#include<iostream>

#include<cstdio>

#include<cstring>

#include<cmath>

#include<algorithm>

#include<queue>

#define re register

#define ll long long

using namespace std;

struct zz

{

    int a,b;

};

int b[],h,t,n,m,x,num,minn=,cnt;

zz d[],q[];

inline bool cmp(zz x,zz y)

{

    return x.a<y.a;

}

int main()

{

    cin>>n>>m;

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

    {

        cin>>x;

        for(re int j=;j<=x;j++)

        {

            cin>>d[++num].a;

            d[num].b=i;

        }

    }

    h=;t=;

    sort(d+,d+num+,cmp);

    for(re int i=;i<=n;i++)

    {

        q[++t]=d[i];

        b[d[i].b]++;

        if(b[d[i].b]==)

        cnt++;

        while(cnt==m)

        {

            minn=min(minn,q[t].a-q[h].a);

            b[q[h].b]--;

            if(b[q[h].b]==)

            cnt--;

            h++;

        }

    }

    cout<<minn;

}