#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <deque>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <cstring>
#include <iomanip>
#include <utility>
 
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,n) for(int i=1;i<=(n);i++)
#define lol long long
#define SUM(n) ((n)+1)*(n)/2　//1〜nまでの総和を求める式
#define mp make_pair
#define fi first
#define se second
#define pu push_back
#define SYOU(x) setprecision(x+1) //小数点桁数を指定する
#define abs(x,y) (max(x,y)-min(x,y))
#define all(v) v.begin(),v.end()
#define UPDight(a,b) (a+b-1)/b //小数点切り上げ 
const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3fLL;
const int MOD=int(1e9)+7; 
using namespace std;
using pii = pair<int,int>;

vector<int> edge[100005];
bool isvisited[100005] = {false};
int dfs(int x,int sum){
  if(isvisited[x]){
    return sum;
  }
  isvisited[x] = true;
  for(int i = 0 ; i < edge[x].size() ; i ++){
    sum = dfs(edge[x][i],sum);
  }
  return sum + 1;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    int a, b;
    rep(i,n - 1){
      cin >> a >> b;
      edge[a].push_back(b);
      edge[b].push_back(a);
    }
    int sum = dfs(1,0);
    if(sum < k){
      cout << -1 << '\n';
    } else {
      cout << k - 1 << '\n';
    }
    return 0;
}