# include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int n,k,le,ri,curt,tim[310],fa[310],que[310],pre[310],mch[310],tmch[310],col[310];
ll mn[310],mx[310];
bool G[310][310];
vector<ll> a[310];
int getF(int x){return fa[x]==x?x:fa[x]=getF(fa[x]);}
int lca(int u,int v)
{
	for(curt++;u;swap(u,v))
		if(tim[u=getF(u)]==curt) return u;
		else tim[u]=curt,u=pre[mch[u]];
	return 0;
}
void blossom(int u,int v,int w)
{
	for(;getF(u)!=w;u=pre[v])
	{
		pre[u]=v;v=mch[u];
		fa[u]=fa[v]=w;
		if(col[v]<0) col[v]=1,que[ri++]=v;
	}
}
bool bfs(int s)
{
	memset(col+1,0,sizeof(int)*(n+1));
	iota(fa+1,fa+n+2,1);
	le=ri=0;que[ri++]=s;col[s]=1;
	while(le<ri)
	{
		int u=que[le++],w;
		for(int v=1;v<=n+1;v++)if(G[u][v])
		{
			if(col[v]<0) continue;
			if(col[v]>0) w=lca(u,v),blossom(u,v,w),blossom(v,u,w);
			else
			{
				pre[v]=u;col[v]=-1;
				if(mch[v]) col[mch[v]]=1,que[ri++]=mch[v];
				else
				{
					for(;v;v=w) u=pre[v],w=mch[u],mch[u]=v,mch[v]=u;
					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+=bfs(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+=bfs(i);
				if(!mch[j]) ans+=bfs(j);
			}
			else if(!mch[i]) ans+=bfs(i);
			if(!mch[n+1]) ans+=bfs(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(judge(mid)) l=mid;
		else r=mid-1;
	}
	cout<<l<<endl;
	return 0;
}