#include <iostream>
using namespace std;

int main(){
    int N,K;
    cin>>N>>K;
    int a[N],b[N];
    int i,j,k;

    for(i=0;i<N-1;i++){
        cin>>a[i]>>b[i];
    }
    if(N<K){
        cout<<"-1"<<endl;
        return 0;
    }
    cout<<K-1<<endl;
    return 0;
}