#include <bits/stdc++.h>
using namespace std;
typedef int64_t ll;

ll n,k;

int main() {
    cin>>n>>k;
    for(ll i=0;i<n-1;i++){
        ll a,b;
        cin>>a>>b;
    }
    cout<<(k>n ? -1 : k-1)<<endl;

    system("pause");
}