#include <bits/stdc++.h>
#define rep(i,n) for (int i=0; i<(n); ++i)
using namespace std;
using ll=long long;
int main(){
    int n,k;
    cin>>n>>k;
    if(n<k){
        cout<<-1<<endl;
        return 0;
    }
    map<int,int>mp;
    mp.insert(make_pair(1,k));
    int ans=0;
    rep(i,n-1){
        int a,b;
        cin>>a>>b;
        if(mp[a]>1){
            mp[a]--;
            mp[b]++;
            ans++;
        }
    }
    cout<<ans<<endl;
}