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

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>

int main() {
  int n, k;
  cin >> n >> k;
  rep(i,n-1) {
    int a, b;
    cin >> a >> b;
  }
  if (n >= k) cout << k-1 << endl;
  else cout << -1 << endl;


}