#include <bits/stdc++.h>

using i64 = long long;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    int n;
    std::cin >> n;
    
    std::vector<std::vector<int>> adj(n);
    for (int i = 1; i < n; i++) {
        int x, y;
        std::cin >> x >> y;
        x--, y--;
        adj[x].push_back(y);
        adj[y].push_back(x);
    }
    
    std::vector<int> c(n);
    for (int i = 0; i < n; i++) {
        std::cin >> c[i];
    }
    
    int ans = 0;
    auto dfs = [&](auto self, int x, int p) -> void {
        for (auto y : adj[x]) {
            if (y == p) {
                continue;
            }
            self(self, y, x);
            if (c[y] == 0) {
                c[y] ^= 1;
                c[x] ^= 1;
                ans++;
            }
        }
    };
    dfs(dfs, 0, -1);
    
    if (c[0] == 0) {
        ans = -1;
    }
    std::cout << ans << "\n";
    
    return 0;
}