#include<iostream>
#include<cstdio>
#include<algorithm>
#define N 300
#define inf 1e18
using namespace std;
long long read()
{
    char c=0;
    int sum=0;
    while (c<'0'||c>'9') c=getchar();
    while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
    return sum;
}
int n,k;
long long a[N+1],ans=inf;
int main()
{
    int t;
    long long x;
    n=read(),k=read();
    for (int i=1;i<=n;++i)
    {
	t=read();
	while (t--) x=read(),a[i]=x;
    }
    if ((n&1)!=(k&1))
    {
	puts("Impossible");
	return 0;
    }
    sort(a+1,a+n+1);
    for (int i=((n+1)>>1);i<=n;++i) ans=min(ans,a[i]-a[i-((n-1)>>1)]);
    printf("%lld\n",ans);
    return 0;
}