#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> PII;
const int maxn=222222,mod=998244353;
#define MP make_pair
#define PB push_back
#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 MEM(x,v) memset(x,v,sizeof(x))
inline ll read(){
	char ch=getchar();ll x=0,f=0;
	while(ch<'0' || ch>'9') f|=ch=='-',ch=getchar();
	while(ch>='0' && ch<='9') x=x*10+ch-'0',ch=getchar();
	return f?-x:x;
}
inline int qpow(int a,int b){
	int ans=1;
	for(;b;b>>=1,a=1ll*a*a%mod) if(b&1) ans=1ll*ans*a%mod;
	return ans;
}
int n,a[maxn],el,head[maxn],to[maxn],nxt[maxn],f[maxn][2][2],tmp[2][2];
inline void add(int u,int v){
	to[++el]=v;nxt[el]=head[u];head[u]=el;
}
void dfs(int u,int F){
	f[u][a[u]][0]=0;
	f[u][a[u]^1][1]=1;
	f[u][a[u]][1]=f[u][a[u]^1][0]=1e9;
	for(int i=head[u];i;i=nxt[i]){
		int v=to[i];
		if(v==F) continue;
		dfs(v,u);
		FOR(a,0,1) FOR(b,0,1) tmp[a][b]=1e9;
		FOR(a,0,1) FOR(b,0,1) FOR(c,0,1) FOR(d,0,1) if(!(b^c)) tmp[a^d][b]=min(tmp[a^d][b],f[u][a][b]+f[v][c][d]);
		FOR(a,0,1) FOR(b,0,1) f[u][a][b]=tmp[a][b];
	}
}
int main(){
	n=read();
	FOR(i,1,n-1){
		int u=read(),v=read();
		add(u,v);add(v,u);
	}
	FOR(i,1,n) a[i]=read();
	dfs(1,0);
	int ans=min(f[1][a[1]][0],f[1][a[1]][1]);
	if(ans>=1e9) puts("impossible");
	else printf("%d\n",ans);
}