/* #region Head */

#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pll = pair<ll, ll>;
template <class T>
using vc = vector<T>;
template <class T>
using vvc = vc<vc<T>>;
using vll = vc<ll>;
using vvll = vvc<ll>;
using vld = vc<ld>;
using vvld = vvc<ld>;
using vs = vc<string>;
using vvs = vvc<string>;
template <class T, class U>
using um = unordered_map<T, U>;

#define REP(i, m, n) for (ll i = (m), i##_len = (ll)(n); i < i##_len; ++(i))
#define REPM(i, m, n) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; ++(i))
#define REPR(i, m, n) for (ll i = (m), i##_min = (ll)(n); i >= i##_min; --(i))
#define REPD(i, m, n, d) for (ll i = (m), i##_len = (ll)(n); i < i##_len; i += (d))
#define REPMD(i, m, n, d) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; i += (d))
#define REPI(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define ALL(x) begin(x), end(x)
#define SIZE(x) ((ll)(x).size())
#define PREM(c)   \
    sort(all(c)); \
    for (bool c##p = 1; c##p; c##p = next_permutation(all(c)))
#define UNIQ(v) v.erase(unique(ALL(v)), v.end());

constexpr ll INF = 1'010'000'000'000'000'017LL;
constexpr ll MOD = 1'000'000'007LL; // 1e9 + 7
constexpr ld EPS = 1e-12;
constexpr ld PI = 3.14159265358979323846;

// vector 入力
template <typename T>
istream &operator>>(istream &is, vc<T> &vec)
{
    for (T &x : vec)
        is >> x;
    return is;
}

// vector 出力 (for dump)
template <typename T>
ostream &operator<<(ostream &os, vc<T> &vec)
{
    ll len = SIZE(vec);
    os << "{";
    for (int i = 0; i < len; i++)
        os << vec[i] << (i == len - 1 ? "" : ", ");
    os << "}";
    return os;
}

// vector 出力 (inline)
template <typename T>
ostream &operator>>(ostream &os, vc<T> &vec)
{
    ll len = SIZE(vec);
    for (int i = 0; i < len; i++)
        os << vec[i] << (i == len - 1 ? "\n" : " ");
    return os;
}

// pair 入力
template <typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &pair_var)
{
    is >> pair_var.first >> pair_var.second;
    return is;
}

// pair 出力
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var)
{
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

// map 出力
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var)
{
    os << "{";
    REPI(itr, map_var)
    {
        os << *itr;
        itr++;
        if (itr != map_var.end())
            os << ", ";
        itr--;
    }
    os << "}";
    return os;
}

// um 出力
template <typename T, typename U>
ostream &operator<<(ostream &os, um<T, U> &map_var)
{
    os << "{";
    REPI(itr, map_var)
    {
        os << *itr;
        auto itrcp = itr;
        itrcp++;
        if (itrcp != map_var.end())
            os << ", ";
    }
    os << "}";
    return os;
}

// set 出力
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var)
{
    os << "{";
    REPI(itr, set_var)
    {
        os << *itr;
        itr++;
        if (itr != set_var.end())
            os << ", ";
        itr--;
    }
    os << "}";
    return os;
}

// dump
#define DUMPOUT cerr
void dump_func()
{
    DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&... tail)
{
    DUMPOUT << head;
    if (sizeof...(Tail) > 0)
    {
        DUMPOUT << ", ";
    }
    dump_func(move(tail)...);
}

// chmax (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>>
bool chmax(T &xmax, const U &x, Comp comp = {})
{
    if (comp(xmax, x))
    {
        xmax = x;
        return true;
    }
    return false;
}

// chmin (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>>
bool chmin(T &xmin, const U &x, Comp comp = {})
{
    if (comp(x, xmin))
    {
        xmin = x;
        return true;
    }
    return false;
}

// ローカル用
#define DEBUG_

#ifdef DEBUG_
#define DEB
#define dump(...)                                                       \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                     \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" \
            << endl                                                     \
            << "    ",                                                  \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct AtCoderInitialize
{
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;

    AtCoderInitialize()
    {
        ios_base::sync_with_stdio(false);
        cin.tie(nullptr);
        cout.tie(nullptr);
        cout << fixed << setprecision(IOS_PREC);
        if (AUTOFLUSH)
            cout << unitbuf;
    }
} ATCODER_INITIALIZE;

/* #endregion */

using Weight = ll;
using Flow = ll;

// エッジ（本来エッジは双方向だが，ここでは単方向で管理）
struct Edge
{
    ll src;        // エッジ始点となる頂点
    ll dst;        // エッジ終点となる頂点
    Weight weight; // 重み
    Flow cap;
    Edge() : src(0), dst(0), weight(0) {}
    Edge(ll src, ll dst, Weight weight) : src(src), dst(dst), weight(weight) {}
};
using Node = vc<Edge>;  // 同じ頂点を始点とするエッジ集合
using Graph = vc<Node>; // graph[i] := 頂点 i を始点とするエッジ集合
// using Array = vector<Weight>;
// using Matrix = vector<Array>;

// 双方向のエッジを追加する
void add_edge(Graph &g, ll a, ll b, Weight w = 1)
{
    g[a].emplace_back(a, b, w);
    g[b].emplace_back(b, a, w);
}
// 単方向のアークを追加する
void add_arc(Graph &g, ll a, ll b, Weight w = 1) { g[a].emplace_back(a, b, w); }

// Edge 標準出力
ostream &operator<<(ostream &os, Edge &edge)
{
    os << "(" << edge.src << " -> " << edge.dst << ", " << edge.weight << ")";
    return os;
}

/**
Problem
 */
void solve()
{
    ll n, k;
    cin >> n >> k;

    vll a(n - 1), b(n - 1);
    REP(i, 0, n - 1)
    {
        cin >> a[i] >> b[i];
        --a[i];
        --b[i];
    }
    Graph graph(n, Node(0));
    REP(i, 0, n - 1)
    {
        add_edge(graph, a[i], b[i]);
    }
    vll dist(n, 0);  // 訪問済み
    queue<Node> que; // 「訪問予定」頂点リスト

    dist[0] = 1;
    que.push(graph[0]);

    // BFS 開始 (キューが空になるまで探索を行う)
    ll cnt = 1;
    while (!que.empty())
    {
        Node current = que.front(); // キューから先頭頂点を取り出す
        que.pop();

        // cuerrent から辿れる頂点をすべて調べる
        for (Edge edge : current)
        {
            if (dist[edge.dst] != 0)
                continue; // 訪問済み (weight=1 固定なら continue してよい)

            // 未訪問の頂点 nv について距離情報を更新してキューに追加する
            dist[edge.dst] = 1;
            ++cnt;
            que.push(graph[edge.dst]);
        }
    }
    if (cnt >= k)
    {
        cout << (k - 1) << endl;
    }
    else
    {
        cout << -1 << endl;
    }
}

/**
 * エントリポイント．
 * @return 0.
 */
int main()
{
    solve();
    return 0;
}