#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>
#include <numeric>
#include <cctype>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

const long long MAX = 5100000;
const long long INF = 1LL << 60;
const long long mod = 1000000007LL;
//const long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;

ll N, K;
vector<int> g[105000];

ll dfs(int cur, int pre, ll h) {
	ll res = 0;
	for (int next : g[cur]) {
		if (next == pre) continue;
		res += dfs(next, cur, h + 1);
	}
	res += h;
	return res;
}
int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/
	scanf("%lld %lld", &N, &K);
	for (int i = 0; i < N - 1; i++) {
		int a, b; scanf("%d %d", &a, &b);
		a--; b--;
		g[a].push_back(b);
		g[b].push_back(a);
	}
	if (N < K) puts("-1");
	else printf("%lld\n", K - 1);
	return 0;
}