#include <bits/stdc++.h>
using namespace std;
using lint     = long long;
const lint inf = 1LL << 60;
const lint mod = 1000000007;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    vector<vector<int>> edges(n);
    for (int i = 0; i < n - 1; ++i) {
        int a, b;
        cin >> a >> b;
        a--, b--;
        edges[a].push_back(b);
        edges[b].push_back(a);
    }
    if (k > n) {
        cout << -1 << "\n";
        return 0;
    } else {
        cout << k - 1 << "\n";
    }

    return 0;
}