#include <bits/stdc++.h>
#define ll long long
#define pii pair<ll,ll>
#define s1 first
#define s2 second
#define TIME chrono::steady_clock::now().time_since_epoch().count()
using namespace std;
int n,k;
ll S=TIME;
vector<ll> vv[311];
ll calc(vector<ll> V)
{
	ll ans=1e18;
	for(int i=0;i+V.size()/2<V.size();++i)ans=min(ans,V[i+V.size()/2]-V[i]);
	return ans;
}
ll solve(vector<ll> V)
{
	sort(V.begin(),V.end());
	int x=n-(k-1);
	ll ans=-1e18;
	for(int i=x/2;i<n-(x/2);++i)
	{
		vector<ll> V1;
		for(int _=0;_<x/2;++_)V1.push_back(V[_]);
		V1.push_back(V[i]);
		for(int _=n-(x/2);_<n;++_)V1.push_back(V[_]);
		ans=max(ans,calc(V1));
	}
	return ans;
}
vector<ll> cV;ll ans=-1e18;
void dfs(int u)
{
	if(u==n+1){ans=max(ans,solve(cV));return;}
	for(int _=0;_<vv[u].size();++_)
	{
		cV.push_back(vv[u][_]);
		dfs(u+1);
		cV.pop_back();
	}
}
ll l[311],r[311];
mt19937 rnd(801);
void poppo()
{
	while((TIME-S)/1e9<4.5)
	{
		for(int x=1;x<=n;++x)
		{
			static vector<pii> vl,vr;
			static bool in[311];
			vl.clear();vr.clear();
			for(int i=1;i<=n;++i)in[i]=0;in[x]=1;
			for(int i=1;i<=n;++i)
			{
				vl.push_back({l[i],i});
				vr.push_back({r[i],i});
			}
			sort(vl.begin(),vl.end());
			sort(vr.begin(),vr.end(),greater<pii>());
			static vector<ll> V;
			V.clear();
			int ptl=0,ptr=0,cntl=0,cntr=0;
			while(cntl<k/2||cntr<k/2)
			{
				while(in[vl[ptl].s2])++ptl;
				while(in[vr[ptr].s2])++ptr;
				if((rnd()&1)&&cntl<k/2||cntr==k/2)
				{
					if(rnd()%100==0)swap(vl[ptl],vl[ptl+1]);
					else
					{
						V.push_back(vl[ptl].s1);
						++cntl,++ptl;
					}
				}
				else
				{
					if(rnd()%100==0)swap(vr[ptr],vr[ptr+1]);
					else
					{
						V.push_back(vr[ptr].s1);
						++cntr;++ptr;
					}
				}
				// if(vl[ptl].s2==vr[ptr].s2&&(rnd()&1))
				// {
				// 	V.push_back(vr[ptr].s1);in[vr[ptr].s2]=1;
				// 	while(in[vl[ptl].s2])++ptl;
				// 	V.push_back(vl[ptl].s1);in[vl[ptl].s2]=1;
				// }
				// else
				// {
				// 	V.push_back(vl[ptl].s1);in[vl[ptl].s2]=1;
				// 	while(in[vr[ptr].s2])++ptr;
				// 	V.push_back(vr[ptr].s1);in[vr[ptr].s2]=1;
				// }
			}
			ll mx=0,mn=1e18;
			for(ll x:V)mx=max(mx,x),mn=min(mn,x);
			ll val=-2e18,ty=-1;
			for(ll y:vv[x])if(min(y-mn,mx-y)>val)val=min(y-mn,mx-y),ty=y;
			V.push_back(ty);
			sort(V.begin(),V.end());
			// printf("V:");for(ll x:V)printf("%lld ",x);putchar(10);
			ans=max(ans,calc(V));
		}
	}
}
int main()
{
	scanf("%d%d",&n,&k);
	int mxt=0;
	for(int i=1;i<=n;++i)
	{
		int t;
		scanf("%d",&t);vv[i].resize(t);
		l[i]=1e18;r[i]=0;
		for(int j=0;j<t;++j)scanf("%lld",&vv[i][j]),mxt=max(mxt,(int)vv[i].size()),l[i]=min(l[i],vv[i][j]),r[i]=max(r[i],vv[i][j]);
	}
	if(n%2!=k%2)printf("Impossible\n");
	else
	{
		if(n<=15||mxt==1)dfs(1);
		else
		{
			k=n-(k-1);
			poppo();
		}
		printf("%lld\n",ans);
	}
	fclose(stdin);fclose(stdout);return 0;
}