#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MAXN = 100005;
const int INF = 1e9;
vector<int> G[MAXN];

int main() {
  int n, k;
  cin >> n >> k;
  for (int i = 0; i < n - 1; ++i) {
    int a, b;
    cin >> a >> b;
    a--; b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }

  if (k < n) {
    cout << k - 1 << '\n';
  } else if (k > n) {
    puts("-1");
  } else {
    cout << n - 1 << '\n';
  }
  return 0;
}