#include <algorithm>

#include  <iterator>

#include  <iostream>

#include   <cstring>

#include   <cstdlib>

#include   <iomanip>

#include    <bitset>

#include    <cctype>

#include    <cstdio>

#include    <string>

#include    <vector>

#include     <stack>

#include     <cmath>

#include     <queue>

#include      <list>

#include       <map>

#include       <set>

#include   <cassert>

/*

⊂_ヽ

　 ＼＼ Λ＿Λ  来了老弟

　　 ＼('ㅅ')

　　　 >　⌒ヽ

　　　/ 　 へ＼

　　 /　　/　＼＼

　　 ﾚ　ノ　　 ヽ_つ

　　/　/

　 /　/|

　(　(ヽ

　|　|、＼

　| 丿 ＼ ⌒)

　| |　　) /

'ノ )　　Lﾉ

*/

using namespace std;

#define lson (l , mid , rt << 1)

#define rson (mid + 1 , r , rt << 1 | 1)

#define debug(x) cerr << #x << " = " << x << "\n";

#define pb push_back

#define pq priority_queue

typedef long long ll;

typedef unsigned long long ull;

//typedef __int128 bll;

typedef pair<ll ,ll > pll;

typedef pair<int ,int > pii;

typedef pair<int,pii> p3;

//priority_queue<int> q;//这是一个大根堆q

//priority_queue<int,vector<int>,greater<int> >q;//这是一个小根堆q

#define fi first

#define se second

//#define endl '\n'

#define boost ios::sync_with_stdio(false);cin.tie(0)

#define rep(a, b, c) for(int a = (b); a <= (c); ++ a)

#define max3(a,b,c) max(max(a,b), c);

#define min3(a,b,c) min(min(a,b), c);

const ll oo = 1ll<<;

const ll mos = 0x7FFFFFFF;  //

const ll nmos = 0x80000000;  //-2147483648

const int inf = 0x3f3f3f3f;

const ll inff = 0x3f3f3f3f3f3f3f3f; //

const int mod = 1e9+;

const double esp = 1e-;

const double PI=acos(-1.0);

const double PHI=0.61803399;    //黄金分割点

const double tPHI=0.38196601;

template<typename T>

inline T read(T&x){

    x=;int f=;char ch=getchar();

    while (ch<''||ch>'') f|=(ch=='-'),ch=getchar();

    while (ch>=''&&ch<='') x=x*+ch-'',ch=getchar();

    return x=f?-x:x;

}

inline void cmax(int &x,int y){if(x<y)x=y;}

inline void cmax(ll &x,ll y){if(x<y)x=y;}

inline void cmin(int &x,int y){if(x>y)x=y;}

inline void cmin(ll &x,ll y){if(x>y)x=y;}

/*-----------------------showtime----------------------*/

            const int maxn = 1e6+;

            pll a[maxn];

            ll d[];

int main(){

            int n,k,tot=;

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

            for(int i=; i<=k; i++){

                int num;    scanf("%d", &num);

                for(int j=; j<=num; j++) {

                    int x;  scanf("%d", &x);

                    a[++tot].fi = x;

                    a[tot].se = i;

                }

            }

            sort(a +, a++tot);

            ll ans = 1ll<<;

            for(int i=; i<=tot; i++){

                d[a[i].se] = a[i].fi;

                ll mx = , flag = , mn = 1ll<<;

                for(int j=; j<=k; j++){

                    mx = max(mx, d[j]);

                    mn = min(mn, d[j]);

                    if(d[j] == ) flag = ;

                }

                if(flag) ans = min(ans, mx - mn);

            }

            printf("%lld\n", ans);

            return ;

}