#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <tuple>
#include <deque>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <complex>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef pair<int, double> Pid;
typedef pair<double, int> Pdi;
typedef pair<ll, int> Pl;
typedef pair<int, pair<int, int>> PP;
constexpr double PI = 3.1415926535897932;   // acos(-1)
constexpr double EPS = 1e-15;
constexpr int INF = 1001001001;
constexpr int mod = 1e+9 + 7;

#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
#define chadd(x, y) x = (x + y) % mod

vector<int> graph[100005];

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, k;
    cin >> n >> k;
    for(int i = 0; i < n - 1; ++i){
        int a, b;
        cin >> a >> b;
        --a, --b;
        graph[a].emplace_back(b);
        graph[b].emplace_back(a);
    }
    if(n < k)   cout << -1 << endl;
    else    cout << k - 1 << endl;
}