#include<iostream>
#include<vector>
using namespace std;
#define ll long long
#define ld long double
#define pii pair<int, int>
#define RUN_LIKE_DJAWAD (ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0));
#define pb push_back
#define pf push_front
#define ff first
#define ss second
#define maxn 100001
vector<int> g[maxn];
int dp[maxn][2][2];
int n, ui, vi;
bool s[maxn], seen[maxn];
void dfs(int v){
seen[v] = true;
int a0 = 0, d0 = 1e9, a1 = 0, d1 = 1e9;
bool c0 = 0, c1 = 0;
for (int u: g[v]) if (not seen[u]){
dfs(u);
a0 += min(dp[u][0][0], dp[u][0][1]);
d0 = min(d0, abs(dp[u][0][0] - dp[u][0][1]));
if (dp[u][0][1] < dp[u][0][0]) c0 ^= 1;
a1 += min(dp[u][1][0], dp[u][1][1]);
d1 = min(d1, abs(dp[u][1][0] - dp[u][1][1]));
if (dp[u][1][1] < dp[u][1][0]) c1 ^= 1;
}
if (c0){
a0 += d0;
d0 *= -1;
}
if (c1){
a1 += d1;
d1 *= -1;
}
if (s[v]){
dp[v][0][0] = a0 + d0;
dp[v][0][1] = a1 + 1;
dp[v][1][0] = a0;
dp[v][1][1] = a1 + d1 + 1;
} else {
dp[v][0][0] = a0;
dp[v][0][1] = a1 + d1 + 1;
dp[v][1][0] = a0 + d0;
dp[v][1][1] = a1 + 1;
}
}
int main(){
RUN_LIKE_DJAWAD
cin >> n;
for (int i = 1; i < n; i++){
cin >> ui >> vi;
g[ui].pb(vi);
g[vi].pb(ui);
}
for (int i = 1; i <= n; i++) cin >> s[i];
dfs(1);
int ans = min(dp[1][0][0], dp[1][0][1]);
if (ans >= 1e9){
cout << "impossible\n";
return 0;
}
cout << ans << "\n";
}
