#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define pb emplace_back
#define lb(v,k) (lower_bound(all(v),(k))-v.begin())
#define ub(v,k) (upper_bound(all(v),(k))-v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T,vector<T>,greater<T>>
#define dame(a) {out(a);return 0;}
#define decimal cout<<fixed<<setprecision(15);
#define all(a) a.begin(),a.end()
#define rsort(a) {sort(all(a));reverse(all(a));}
#define dupli(a) {sort(all(a));a.erase(unique(all(a)),a.end());}
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
using vi=vector<ll>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vvvvi=vector<vvvi>;
using vp=vector<P>;
using vvp=vector<vp>;
using vb=vector<bool>;
using vvb=vector<vb>;
const ll inf=1001001001001001001;
const ll INF=1001001001;
const ll mod=1000000007;
const double eps=1e-10;
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yesno(T b){if(b)out("yes");else out("no");}
template<class T> void YesNo(T b){if(b)out("Yes");else out("No");}
template<class T> void YESNO(T b){if(b)out("YES");else out("NO");}
template<class T> void outset(T s){auto itr=s.begin();while(itr!=s.end()){if(itr!=s.begin())cout<<' ';cout<<*itr;itr++;}cout<<'\n';}
void outs(ll a,ll b){if(a>=inf-100)out(b);else out(a);}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){ll res=1;a%=mod;while(b){if(b&1)res=res*a%mod;a=a*a%mod;b>>=1;}return res;}
class BIT{
vi bit;ll N;
public:
ll sum(ll i){
chmin(i,N);
ll res=0;
for(ll j=i-1;j>=0;j=(j&(j+1))-1)res+=bit[j];
return res;
}
BIT(ll n):N(n){bit.resize(N);}
void add(ll i,ll x){
for(int j=i;j<N;j|=j+1)bit[j]+=x;
}
ll get(ll a,ll b){
return sum(b)-sum(a);
}
};
class UF{
vi data;stack<P> st;
public:
UF(ll n){
data=vi(n,1);
}
bool merge(ll x,ll y,bool undo=false){
x=root(x);y=root(y);
if(data[x]>data[y])swap(x,y);
if(undo){st.emplace(y,data[y]);st.emplace(x,data[x]);}
if(x==y)return false;
data[y]+=data[x];data[x]=-y;
return true;
}
ll root(ll i){if(data[i]>0)return i;return root(-data[i]);}
ll getsize(ll i){return data[root(i)];}
bool same(ll a,ll b){return root(a)==root(b);}
void undo(){rep(i,2){data[st.top().fi]=st.top().se;st.pop();}}
};
int main(){
ll n;cin>>n;
vvi g(n);
rep(i,n-1){
ll a,b;cin>>a>>b;a--;b--;
g[a].pb(b);g[b].pb(a);
}
vi v(n);rep(i,n)cin>>v[i];
vvvi dp(n,vvi(2,vi(2,inf)));
function<void(ll,ll)> dfs=[&](ll i,ll p){
for(ll x:g[i])if(x!=p)dfs(x,i);
rep(c,2){
ll cnt=0,sum=0,dif=inf;
for(ll x:g[i])if(x!=p){
sum+=min(dp[x][c][0],dp[x][c][1]);
if(dp[x][c][1]<dp[x][c][0])cnt++;
chmin(dif,abs(dp[x][c][0]-dp[x][c][1]));
}
chmin(dp[i][(c+v[i]+cnt)&1][c],sum+c);
chmin(dp[i][(c+v[i]+cnt+1)&1][c],sum+c+dif);
}
// out(i);outvv(dp[i]);
};dfs(0,-1);
ll ans=inf;
rep(i,2)chmin(ans,dp[0][0][i]);
if(ans==inf)out("impossible");
else out(ans);
}
Compilation message
/usr/bin/ld: /tmp/ccYDMmGw.o: in function `main':
grader.cpp:(.text.startup+0x0): multiple definition of `main'; /tmp/ccbcbEty.o:swaps.cpp:(.text.startup+0x0): first defined here
/usr/bin/ld: /tmp/ccYDMmGw.o: in function `main':
grader.cpp:(.text.startup+0x67): undefined reference to `solve(int, int)'
collect2: error: ld returned 1 exit status
