# include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int n,k,mch[310],tmch[310];
ll mn[310],mx[310];
bool G[310][310],vis[310];
vector<ll> a[310];
mt19937 Rand(0);
bool dfs(int u)
{
	if(vis[u]) return false;
	vis[u]=1;
	vector<int> vec;
	for(int v=1;v<=n+1;v++)
		if(G[u][v] && !vis[v]) vec.push_back(v);
	shuffle(vec.begin(),vec.end(),Rand);
	for(int v:vec)
	{
		int w=mch[v];
		mch[u]=v;mch[v]=u;mch[w]=0;
		if(!w || dfs(w)) return true;
		mch[u]=0;mch[v]=w;mch[w]=v;
	}
	return false;
}
bool aug(int u)
{
	for(int T=0;T<5;T++)
	{
		bool ok=dfs(u);
		memset(vis+1,0,sizeof(int)*(n+1));
		if(ok) return true;
	}
	return false;
}
bool judge(ll lim)
{
	int ans=0,tans=0;
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)if(i!=j)
			G[i][j]=(mx[i]-mn[j]>=lim || mx[j]-mn[i]>=lim);
	for(int i=1;i<=n;i++) G[i][n+1]=G[n+1][i]=0;
	memset(mch+1,0,sizeof(int)*(n+1));
	for(int i=1;i<=n;i++)
		if(!mch[i]) ans+=aug(i);
	for(int i=1;i<=n;i++)for(ll v:a[i])
	{
		int d1=0,d2=0,v1=0,v2=0;
		for(int j=1;j<=n;j++)if(i!=j)
		{
			G[i][j]=G[j][i]=(mn[j]<=v-lim);
			G[n+1][j]=G[j][n+1]=(mx[j]>=v+lim);
			if(G[i][j]) d1++,v1=j;
			if(G[n+1][j]) d2++,v2=j;
		}
		if(d1 && d2 && (d1>1 || d2>1 || v1!=v2))
		{
			tans=ans;memcpy(tmch+1,mch+1,sizeof(int)*(n+1));
			if(mch[i] && !G[i][mch[i]])
			{
				int j=mch[i];mch[i]=mch[j]=0;ans--;
				if(!mch[i]) ans+=aug(i);
				if(!mch[j]) ans+=aug(j);
			}
			else if(!mch[i]) ans+=aug(i);
			if(!mch[n+1]) ans+=aug(n+1);
			if(ans>=k) return true;
			ans=tans;memcpy(mch+1,tmch+1,sizeof(int)*(n+1));
		}
		for(int j=1;j<=n;j++)if(i!=j)
			G[i][j]=G[j][i]=(mx[i]-mn[j]>=lim || mx[j]-mn[i]>=lim),
			G[n+1][j]=G[j][n+1]=0;
	}
	return false;
}
int main()
{
	cin>>n>>k;
	if((n-k)&1) return puts("Impossible"),0;
	k=(n-k+2)/2;
	for(int i=1;i<=n;i++)
	{
		int x;
		scanf("%d",&x);a[i].resize(x);
		for(ll &j:a[i]) scanf("%lld",&j);
		sort(a[i].begin(),a[i].end());
		mn[i]=a[i][0];mx[i]=a[i][x-1];
	}
	ll l=0,r=1e18,mid;
	while(l<r)
	{
		mid=(l+r+1)/2;
		if(n>20) puts("OK");
		if(judge(mid)) l=mid;
		else r=mid-1;
	}
	cout<<l<<endl;
	return 0;
}