#include <iostream>
#include <vector>
#include <climits>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <string>
#include <bitset>
#include <utility>
#include <numeric>
#include <queue>
#include <stack>

using ll = long long;
using namespace std;

constexpr int MOD = 1e9 + 7;
constexpr ll MOD_LL = ll(1e9) + 7;

int main(void) {
	int n, k;
	cin >> n >> k;
	
	vector< vector<int> > t(n);
	for(int i = 0; i < n - 1; ++i) {
		int a, b;
		cin >> a >> b;
		a--; b--;
		t[a].push_back(b);
		t[b].push_back(a);
	}
	
	if( n < k ) {
		cout << -1 << endl;
	} else {
		cout << k - 1 << endl;
	}
	
	return 0;
}