#include <bits/stdc++.h>

using namespace std;

#define lli long long int
#define REP(i,s,n) for(int i=s;i<n;i++)
#define MOD 1000000007
#define NUM 2520
#define INF (1LL<<50)
#define DEBUG 1
#define mp(a,b) make_pair(a,b)
#define SORT(V) sort(V.begin(),V.end())
#define PI (3.141592653589794)

signed main(){

	lli n,k;
	cin>>n>>k;

	vector<vector<lli>> v(n);

	REP(i,0,n-1){
		lli a,b;
		cin>>a>>b;
		a--,b--;
		v[a].push_back(b);
		v[b].push_back(a);
	}

	if(k>n){
		cout<<-1<<endl;
		return 0;
	}

	cout<<k-1<<endl;


	return 0;
}