#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;

void dfs(const Graph &g, vector<bool> &visit, int v) {
    if (visit[v])
        return;
    visit[v] = true;

    for (auto e : g[v]) {
        dfs(g, visit, e);
    }
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

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

    if (k >= n) {
        cout << -1 << endl;
        return 0;
    }

    vector<bool> visit(n, false);
    dfs(g, visit, 0);

    for (int i = 0; i < n; i++) {
        if (!visit[i]) {
            cout << -1 << endl;
            return 0;
        }
    }

    cout << (k - 1) << endl;

    return 0;
}