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#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define all(x) x.begin(),x.end()
#define CLR(i,a) memset(i,a,sizeof(i))
#define fi first
#define se second
#define pb push_back
#define SZ(a) ((int) a.size())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
typedef array<ll,3> arr;
typedef double DB;
typedef pair<DB,DB> pdd;
constexpr ll MAXN=static_cast<ll>(2e5)+10,INF=static_cast<ll>(5e18)+3;
typedef pair<ll,bool> plb;
ll N,M,T,A[MAXN];
vector<ll> g[MAXN];
ll ans=0;
bool vis[MAXN];
ll dp[MAXN];
pair<ll,bool> dfs(ll x){
ll lans=0;
bool flag=false;
vector<ll> rett;
FOR(i,0,SZ(g[x])-1){
ll node=g[x][i];
if(!vis[node]){
vis[node]=true;
plb ret=dfs(node);
if(ret.se) flag=true;
lans+=ret.fi;
rett.pb(ret.fi);
}
}
if(SZ(g[x])==4){
lans+=1;
ans=max(lans,ans);
return (plb){lans,true};
}else if(SZ(g[x])==1){
lans+=1;
if(flag) ans=max(lans,ans);
return (plb){lans,flag};
}else if(SZ(g[x])<4){
if(flag){
sort(all(rett));
ans=max(ans,1+rett[SZ(rett)-1]);
}
return (plb){1,false};
}else{
sort(all(rett));
lans=0;
ROF(i,SZ(rett)-1,SZ(rett)-3){
lans+=rett[i];
}
lans+=1;
ans=max(ans,lans+rett[SZ(rett)-4]);
return (plb){lans,true};
}
}
inline void solve(){
cin>>N;
ans=0;
FOR(i,1,N-1){
ll a,b;
cin>>a>>b;
g[a].pb(b);
g[b].pb(a);
}
vis[1]=true;
dfs(1);
cout<<(ans==0?-1:ans)<<"\n";
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
solve();
return 0;
}
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