#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;
}

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5+5;
struct E {
	int nxt,to;
}e[N<<1];
int head[N],tot;
void add(int x,int y) {
	e[++tot].nxt = head[x];
	head[x] =  tot;
	e[tot].to = y;
}
int dis1[N],dis2[N],fa[N];
void dfs1(int x,int f) {
	fa[x] = f;
	if (x!=f)dis1[x] = dis1[f]+1;
	for (int i = head[x];i;i=e[i].nxt) 
		if (e[i].to!=f)
			dfs1(e[i].to,x);
}
void dfs2(int x,int f) {
	if (x!=f)dis2[x] = dis2[f]+1;
	for (int i = head[x];i;i=e[i].nxt) 
		if (e[i].to!=f)
			dfs2(e[i].to,x);
}
int main() {
	int n,t;
	scanf("%d%d",&n,&t);
	int x,y;
	for (int i = 1;i< n;i++) {
		scanf("%d%d",&x,&y);
		add(x,y);add(y,x);
	}
	dfs1(1,1);
	x = n;
	while (dis1[x] > t)
		x = fa[x];
	int ans = 0;
	dis1[x] = 0;
	dfs1(x,x);
	dis2[n] = 0;
	dfs2(n,n);
	for (int i = 1;i<= n;i++)
		if (2*dis1[i] <= dis2[i])
			ans = max(ans,((dis2[i]+1)>>1));
	printf("%d\n",ans);
	return 0;
}