#include "bits/stdc++.h"
using namespace std;
#ifdef DEBUG
auto&operator<<(auto &o, pair<auto, auto> p) {o << "(" << p.first << ", " << p.second << ")"; return o;}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{"; for(auto e : x) o<<e<<", "; return o<<"}";}
#define debug(X) cerr << "["#X"]: " << X << '\n';
#else
#define cerr if(0)cout
#define debug(X) ;
#endif
using ll = long long;
#define all(v) (v).begin(), (v).end()
#define ssize(x) int(x.size())
#define fi first
#define se second
#define mp make_pair
#define eb emplace_back
const int inf = 1e9;
struct node {
int dp[2][2];
node() {
dp[0][0] = dp[0][1] = dp[1][0] = dp[1][1] = inf;
}
node(int x) {
dp[1-x][0] = dp[1-x][1] = 1;
dp[x][1] = 0;
dp[x][0] = inf;
}
friend ostream& operator << (auto &os, const node &a) {
return os << "{{" << a.dp[0][0] << ", " << a.dp[0][1] << "}, {"
<< a.dp[1][0] << ", " << a.dp[1][1] << "}}";
}
};
node comb(const node &a, const node &b) {
node res;
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j) {
for (int x = 0; x < 2; ++x) {
for (int y = 0; y < 2; ++y) {
if (j && y) continue;
int col = i, add = 0;
if (!x) {
col ^= 1;
add += 2;
}
if (y) {
res.dp[col][1] = min(res.dp[col][1], a.dp[i][j] + b.dp[x][y] + add);
}
else {
if (!j)
res.dp[col][1] = min(res.dp[col][1], a.dp[i][j] + b.dp[x][y] + add);
add++;
col ^= 1;
res.dp[col][j] = min(res.dp[col][j], a.dp[i][j] + b.dp[x][y] + add);
}
}
}
}
}
return res;
}
vector<node> edge_dp(const vector<int> &u, const vector<int> &v,
const vector<vector<int>> &g, const vector<int> &c) {
int n = ssize(g);
int m = 2 * (n-1);
vector<bool> skip(m);
vector<node> dp(m);
for (int i = 0; i < m; ++i) if (c[v[i]]) skip[i] = true;
for (int i = 0; i < m; ++i) dp[i] = node(c[v[i]]);
vector<vector<int>> rev(n);
for (int i = 0; i < m; ++i)
rev[v[i]].eb(i);
stack<int> s;
vector<int> st(m);
for (int i = 0; i < m; ++i) {
st[i] = ssize(g[v[i]]) - 1;
if (!st[i]) s.push(i);
}
while (ssize(s)) {
int e = s.top(); s.pop();
debug(e);
for (int idx : g[v[e]]) {
if (idx == (e^1)) continue;
if (!skip[idx]) {
dp[e] = comb(dp[e], dp[idx]);
skip[e] = false;
}
}
debug(skip[e]);
debug(dp[e]);
for (int idx : rev[u[e]]) {
if (idx == (e^1)) continue;
if (--st[idx] == 0) s.push(idx);
}
}
debug(dp);
vector<node> dp_v(n);
for (int i = 0; i < n; ++i) dp_v[i] = node(c[i]);
for (int i = 0; i < n; ++i) {
for (int idx : g[v[i]]) {
if (!skip[idx]) {
dp_v[i] = comb(dp_v[i], dp[idx]);
}
}
}
debug(dp_v);
return dp_v;
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n;
cin >> n;
vector<int> c(n);
vector<vector<int>> g(n);
for (int i = 0; i < n; ++i) {
char x;
cin >> x;
c[i] = int(x - '0');
}
vector<int> u(2*n-2), v(2*n-2);
for (int i = 0; i < n-1; ++i) {
int a, b;
cin >> a >> b;
--a; -- b;
u[2*i] = v[2*i+1] = a;
v[2*i] = u[2*i+1] = b;
g[u[2*i]].eb(2*i);
g[u[2*i+1]].eb(2*i+1);
}
vector<node> dp = edge_dp(u, v, g, c);
int res = inf;
for (int i = 0; i < n; ++i)
res = min(res, min(dp[i].dp[1][0], dp[i].dp[1][1]));
cout << res << '\n';
return 0;
}
