#include<bits/stdc++.h>
using namespace std;
#define int long long
const int NR=310;
int n,k,cnt[NR],v[NR],vis[NR],len;
mt19937 rnd(time(0));
int Rand(int l,int r){return rnd()%(r-l+1)+l;}
struct task{
	int val,x,y;
	bool operator <(const task &T)const{
		return val<T.val;
	}
}t[NR<<3],p[NR<<3];
vector<int>a[NR];

bool check(int x){
	int tar=(n-k+1),tot=0;
	memset(vis,0,sizeof(vis));
	for(int i=1;i<=len;i++)
		if(!vis[p[i].x]){
			if(tot<tar/2)v[++tot]=p[i].val,vis[p[i].x]=1;
			else if(p[i].val-v[tot-tar/2+1]>=x)v[++tot]=p[i].val,vis[p[i].x]=1;
		}
	return tot>=tar;
}
bool Check(int x){
	for(int T=1;T<=100;T++){
		for(int i=1;i<=len;i++)p[i]=t[i];
		int now=0;
		while(now<=len){
			int k=Rand(1,min(10ll,len-now));
			if(Rand(1,10)==1)shuffle(p+now+1,p+now+k+1,rnd);
			now+=k;
		}
		if(check(x))return 1;
	}
	return 0;
}

signed main(){
	cin>>n>>k;
	if((n+k)&1){
		puts("Impossible");
		return 0;
	}
	for(int i=1;i<=n;i++){
		cin>>cnt[i];a[i].resize(cnt[i]);
		for(int j=0;j<cnt[i];j++)cin>>a[i][j],t[++len]=task{a[i][j],i,j};
		sort(a[i].begin(),a[i].end());
	}
	sort(t+1,t+1+len);
	int l=0,r=1e18,res=0;
	while(l<=r){
		int mid=(l+r)>>1;
		if(check(mid))res=mid,l=mid+1;
		else r=mid-1;
	}
	cout<<res<<endl;
	return 0;
}