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

struct UnionFind {
  vector<int> data;
  UnionFind(int size) : data(size, -1) { }
  bool unionSet(int x, int y) {
    x = root(x); y = root(y);
    if (x != y) {
      if (data[y] < data[x]) swap(x, y);
      data[x] += data[y]; data[y] = x;
    }
    return x != y;
  }
  bool findSet(int x, int y) {
    return root(x) == root(y);
  }
  int root(int x) {
    return data[x] < 0 ? x : data[x] = root(data[x]);
  }
  int size(int x) {
    return -data[root(x)];
  }
};

int main()
{
	int N, K;
	cin >> N >> K;
	UnionFind uf( N );
	for( int i = 0; i < N - 1; i++ ) {
		int a, b;
		cin >> a >> b;
		a--;
		b--;
		uf.unionSet( a, b );
	}

	int ans = -1;
	if( K <= uf.size( 0 ) ) ans = K - 1;

	cout << ans << endl;
}