#include <bits/stdc++.h>
using namespace std;

#define int long long

vector<int> tree[100010];

signed main(){
	int n, k;
	cin >> n >> k;
	for(int i = 0;i < n-1;i++){
		int a, b;
		cin >> a >> b;
		a--; b--;
		tree[a].push_back(b);
		tree[b].push_back(a);
	}

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

	return 0;
}