#include <bits/stdc++.h>
const int N = 1e6 + 50;
std::vector<int> G[N];
int a[N], dp[N][2][2], g[N][2][2], h[2][2];
void dfs(int x, int fa = 0) {
memset(g[x], 0x3f, sizeof g[x]);
g[x][0][0] = g[x][1][0] = 0;
for (int y : G[x])
if (y != fa) {
dfs(y, x);
memcpy(h, g[x], sizeof h);
for (int o : {0, 1})
for (int p : {0, 1})
g[x][o][p] = std::min(0x3f3f3f3f, std::min(h[o][p] + dp[y][0][o], h[o][!p] + dp[y][1][o]));
}
for (int o : {0, 1})
for (int p : {0, 1})
dp[x][o][a[x] ^ o ^ p] = g[x][o][p] + o;
}
int main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
std::cout.tie(nullptr);
int n;
std::cin >> n;
for (int i = 1; i < n; ++i) {
int x, y;
std::cin >> x >> y;
G[x].push_back(y);
G[y].push_back(x);
}
for (int i = 1; i <= n; ++i) std::cin >> a[i];
memset(dp, 0x3f, sizeof dp);
dfs(1);
int ans = std::min(dp[1][0][0], dp[1][1][0]);
if (ans > n) {
puts("impossible");
}
else {
printf("%d\n", ans);
}
return 0;
}
