#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <optional>
#include <queue>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    int32_t n, k;
    cin >> n >> k;
    vector<vector<int32_t>> tr(n);
    for (auto i = 0; i < n - 1; ++i) {
        int32_t a, b;
        cin >> a >> b;
        --a;
        --b;
        tr[a].push_back(b);
        tr[b].push_back(a);
    }
    cout << (k <= n ? k - 1 : -1) << endl;
    return 0;
}