#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<utility>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<functional>
#include<math.h>
#include<random>
#include <bitset>
using namespace std;
#define N (1000000000+7)
//#define N 998244353
#define INF 1e16
typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,P> Q;
typedef vector<ll> vec;
typedef vector<vec> mat;

const int inf = (int)1e9; 

int main(void){
	int n,k;
	cin>>n>>k;
	for(int i=0;i<n-1;i++){
		int a,b;
		cin>>a>>b;
	}
	if(n<k)cout<<-1<<endl;
	else cout<<k-1<<endl;
	return 0;
}