#include <iostream>
#include <string>

using namespace std;

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;
}