#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define repr(i, n) for (int i = (n) - 1; i >= 0; i--)
#define range(a) a.begin(), a.end()

int main() {
  int N, K;
  cin >> N >> K;
  vector<vector<int>> G(N);
  rep(i, N - 1) {
    int u, v; cin >> u >> v; u--; v--;
    G[u].push_back(v);
    G[v].push_back(u);
  }
  if (K > N) {
    cout << "-1" << endl;
    return 0;
  }
  cout << K - 1 << endl;
}