#include <iostream>
#include <iomanip>
#include <vector>
#include <utility>
#include <map>
#include <algorithm>
#include <queue>
#include <cmath>
#include <numeric>
#include <set>

using namespace std;
struct aaa{aaa(){cin.tie(nullptr); ios::sync_with_stdio(false); cout<<fixed<<setprecision(20);};}aaa;
template <class T>ostream &operator<<(ostream &o,const vector<T>&v){o<<"{";for(int i=0;i<(int)v.size();i++)o<<(i>0?", ":"")<<v[i];o<<"}";return o;}
#define debug(v) {cerr<<"\033[1;36m[debug]\033[m "<<#v<<" : "<<v<<endl;}

using int64 = long long;
using P = pair<int,int>;

void bfs(int& n, int& k, const vector<vector<int>>& g) {
    queue<P> q;
    q.emplace(1,0);

    while(!q.empty() && k > 0) {
        int u,i;
        tie(u,i) = q.front(); q.pop();

        k--;
        for (auto v : g[u]) {
            q.emplace(v, i+1);
        }
    }
}

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

    int ans = k-1;
    bfs(n, k, g);

    cout << (k>0? -1 : ans) << endl;
}