#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
// Welcome to my source code!

int main() {
    int n, k;
    cin >> n >> k;
    int v[n] = {};
    for (int i = 0, a, b; i < n - 1; i++) {
        cin >> a >> b;
        v[a - 1] = b - 1;
    }/*
    bool flag = false;
    int cur = 0;
    for (int i = 0; i < n; i++) {
        v[cur] = v[i]
    }
    if (flag) cout << k - 1 << endl;
    else cout << -1 << endl;
    */
    if (n >= k) cout << k - 1 << endl;
    else cout << -1 << endl;
}