#include <bits/stdc++.h>

using namespace std;

int n;
vector<vector<int>> G;
vector<int> c;
int ans = 0;

int dfs(int pos, int pre) {
    int cnt = 0;
    for (auto u : G[pos]) {
        if (u != pre) {
            cnt += dfs(u, pos);
        }
    }
    if ((cnt + c[pos]) % 2 == 0) {
        ans++;
        return 1;
    }
    return 0;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n;
    G.resize(n);
    c.resize(n);
    for (int i = 0; i < n - 1; i++) {
        int a, b;
        cin >> a >> b;
        a--, b--;
        G[a].push_back(b);
        G[b].push_back(a);
    }
    for (int i = 0; i < n; i++) {
        cin >> c[i];
    }
    cout << (dfs(0, -1) == 0 ? ans : -1) << endl;
}