#include<bits/stdc++.h>
#define int ll
#define lc (rt<<1)
#define rc (rt<<1)
#define mid (l+r>>1)
#define rg register
#define pb push_back
#define ppb pop_back
#define mkp make_pair
#define DEBUG printf("Debug on line %d\n", __LINE__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
const int N = 1e5 + 5;
const int INF = 0x3f3f3f3f;
inline int read(){
int n=0, f=1; char ch = getchar();
for(; !isdigit(ch); ch=getchar()) if(ch == '-') f=-1;
for(; isdigit(ch); ch=getchar()) n=n*10+ch-'0';
return n*f;
}
struct edge{
int nxt;
int to;
}g[N<<1];
int n;
int head[N], cnt;
int a[N];
int f[N][2][2];
void add_edge(int u, int v){
g[++cnt].nxt = head[u];
g[cnt].to = v;
head[u] = cnt;
}
void dfs(int u, int fa){
f[u][0][0] = 0;
f[u][1][1] = 1;
f[u][0][1] = f[u][1][0] = INF;
for(int i=head[u]; i; i=g[i].nxt){
int v = g[i].to;
if(v == fa) continue;
dfs(v, u);
int x = min(f[u][0][0] + f[v][0][a[v]], f[u][0][1] + f[v][1][a[v]]);
int y = min(f[u][0][1] + f[v][0][a[v]], f[u][0][0] + f[v][1][a[v]]);
int z = min(f[u][1][0] + f[v][0][!a[v]], f[u][1][1] + f[v][1][!a[v]]);
int w = min(f[u][1][1] + f[v][0][!a[v]], f[u][1][0] + f[v][1][!a[v]]);
f[u][0][0] = x, f[u][0][1] = y, f[u][1][0] = z, f[u][1][1] = w;
}
// printf("%d f00 = %d, f01 = %d, f10 = %d, f11 = %d\n", u, f[u][0][0], f[u][0][1], f[u][1][0], f[u][1][1]);
}
signed main(){
n = read();
for(int i=1; i<n; i++){
int u = read(), v = read();
add_edge(u, v);
add_edge(v, u);
}
for(int i=1; i<=n; i++){
a[i] = read();
}
dfs(1, 0);
int ans = min(f[1][0][a[1]], f[1][1][a[1]]);
if(ans >= INF) printf("impossible\n");
else printf("%lld\n", ans);
}
