#include <bits/stdc++.h>
using namespace std;

const int N = 1e5 + 5;
const int inf = 0x3f3f3f3f;

int n;
int a[N], dp[N][2][2], f[2][2];
vector<int> e[N];

void dfs(int u, int fa) {
	dp[u][a[u]][0] = 0;
	dp[u][a[u] ^ 1][1] = 1;
	for(int v : e[u]) {
		if(v == fa) {
			continue;
		}
		dfs(v, u);
		f[0][0] = min(dp[u][0][0] + dp[v][0][0], dp[u][1][0] + dp[v][0][1]);
		f[0][1] = min(dp[u][0][1] + dp[v][1][0], dp[u][1][1] + dp[v][1][1]);
		f[1][0] = min(dp[u][0][0] + dp[v][0][1], dp[u][1][0] + dp[v][0][0]);
		f[1][1] = min(dp[u][0][1] + dp[v][1][1], dp[u][1][1] + dp[v][1][0]);
		for(int i = 0; i < 2; i++) {
		    for(int j = 0; j < 2; j++) {
		        dp[u][i][j] = min(f[i][j], 0x3f3f3f3f);
		    }
		}
	}
}

int main() {
	ios::sync_with_stdio(0); cin.tie(0);
	cin >> n;
	for (int i = 1; i < n; i++) {
		int u, v;
		cin >> u >> v;
		e[u].emplace_back(v);
		e[v].emplace_back(u);
	}
	for(int i = 1; i <= n; i++) {
		cin >> a[i];
	}
	memset(dp, 0x3f, sizeof(dp));
	dfs(1, 0);
	int ans = min(dp[1][0][0], dp[1][0][1]);
	if(ans == inf) {
	    cout << "impossible\n";
	}
	else {
	    cout << ans << '\n';
	}
	
	return 0;
}