#include <bits/stdc++.h>
using namespace std;
const int N = 1e6+5;
struct Node {
	int val,bel;
}clo[N<<1];
bool cmp(const Node &x,const Node &y) { return x.val < y.val; }
int buk[N],cnt,n,m;
int main()
{
	scanf("%d%d",&n,&m);
	int ki;
	for (int i = 1;i<= n;i++) {
		scanf("%d",&ki);
		for (int j = 1;j<= ki;j++) {
			cnt++;
			scanf("%d",&clo[cnt].val);
			clo[cnt].bel = i;
		}
	}
	sort(clo+1,clo+cnt+1,cmp);
	for (int i = 1;i<= n;i++)buk[i] = 0;
	int tail = 0;
	int ans = 1e9+5;
	int cntbel = 0;
	for (int i = 1;i<= cnt;i++) {
		while (tail+1<=cnt) {
			tail++;
			buk[clo[tail].bel]++;
			if (buk[clo[tail].bel] == 1)cntbel++;
			if (cntbel >= m) {ans = min(ans,clo[tail].val-clo[i].val);break;}
		}
		buk[clo[i].bel]--;
		if (buk[clo[i].bel]==0)cntbel--;
	}
	printf("%d\n",ans);
	return 0;
}