#include <algorithm>
#include <iostream>
#include <string>

using namespace std;

int main() {
    int n, k;
    cin >> n >> k;

    int a[n-1], b[n-1];
    for (int i = 0; i < n-1; i++) {
        cin >> a[i] >> b[i];
    }

    if (n == k) {
        cout << (k - 1);
    } else {
        cout << -1;
    }
    cout << endl;

    return 0;
}