#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define printVec(v) printf("{"); for (const auto& i : v) { std::cout << i << ", "; } printf("}\n");
#define degreeToRadian(deg) (((deg)/360)*2*M_PI)
#define radianTodegree(rad) (((rad)/2/M_PI)*360)
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; }
using namespace std;
using P = pair<int,int>;
using ll = long long;
const ll INF = 1LL<<60;
const int MOD = 1e9 + 7;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

//cin.tie(0);ios::sync_with_stdio(false);
//cout << fixed << setprecision(15) << y << endl;
struct UnionFind {
    vector<int> a;
    UnionFind(int n) : a(n) {
        for(int i = 0; i < n; i++) a[i] = i;
    }

    int root(int x) { 
        if (a[x] == x) return x;
        return a[x] = root(a[x]);
    }

    void unite(int x, int y) {
        int rx = root(x);
        int ry = root(y);
        if (rx == ry) return;
        a[rx] = ry;
    }

    bool same(int x, int y) {
        int rx = root(x);
        int ry = root(y);
        return rx == ry;
    } 
};

int main() {
  int n, k;
  cin >> n >> k;
  UnionFind uf(n);
  rep (i, n - 1) {
    int a, b;
    cin >> a >> b;
    --a; --b;
    uf.unite(a, b);
  }

  int cnt = 0;
  rep (i, n) if (uf.same(0, i)) cnt++;

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