#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <fstream>
#include <string>
#include <set>
#include <deque>
#include <queue>
#include <map>
#define int  long long
#define endl "\n"
using namespace std;
const int INF = 1e18;
long double eps = 1e-9;
vector<vector<int>> g;
vector<int> color;
int n, x, y, k, res;
int DFS(int i, int p) {
    color[i] = 1;
    int maxd = -1;
    for (int v : g[i]) {
        if (v == p) {
            continue;
        }
        if (color[v] == 0) {
            int d = DFS(v, i);
            if (maxd != -1) {
                res = max(res, d + maxd + 2);
            }
            maxd = max(maxd, d);
            res = max(res, maxd + 1);
        }
    }
    return maxd + 1;
}
signed main() {
    cin >> n;
    int i, j, st = INF;
    bool sup = false;
    vector <int> check(n);
    for (i = 0; i < n; i++) {
        int a;
        cin >> a;
        if (a == 1 && st == INF) {
            st = i;
        }
        check[i] = a;
    }
    g.resize(n);
    color.resize(n);
    for (i = 0; i < n - 1; i++) {
        cin >> x >> y;
        x--;
        y--;
        g[x].push_back(y);
        g[y].push_back(x);
    }
    /*
    for (i = 0; i < n; i++) {
        for (j = 0; j < g[i].size(); j++) {
            cout << g[i][j] << " ";
        }
        cout << endl;
    }
    */
    DFS(0, -1);
    cout << (res + 2 - 1) / 2;
}