#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using Graph=vector<vector<pair<int,int>>>;
#define INF 1000000000000000000
#define MOD 1000000007

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

  if(K>N){
    cout<<-1<<endl;
  }else{
    cout<<K-1<<endl;
  }
}