#include <bits/stdc++.h>

using namespace std;
using ll=long long;

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