#include "bits/stdc++.h"
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;

typedef long long ll;

int main() {
    int N, K, a, b;
    cin >> N >> K;
    rep(i, N - 1) cin >> a >> b;
    if (K > N) cout << "-1\n";
    else cout << K - 1 << endl;
}