#include <iostream>
#include <string>
#include <cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<iomanip>
#define _USE_MATH_DEFINES
#include <math.h>
#include <functional>
#include<complex>
using namespace std;

#define rep(i,x) for(ll i=0;i<x;i++)
#define repn(i,x) for(ll i=1;i<=x;i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;
const double eps = 1E-18;

ll max(ll a, ll b) {
	if (a > b) { return a; }
	return b;
}

ll min(ll a, ll b) {
	if (a > b) { return b; }
	return a;
}

ll gcd(ll a, ll b) {
	if (b == 0) { return a; }
	if (a < b) { return gcd(b, a); }
	return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

struct edge {
	ll ind;
	ll fr;
	ll to;
	ll d;
};

class mint {
	long long x;
public:
	mint(long long x = 0) : x((x% MOD + MOD) % MOD) {}
	mint operator-() const {
		return mint(-x);
	}
	mint& operator+=(const mint& a) {
		if ((x += a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator-=(const mint& a) {
		if ((x += MOD - a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator*=(const  mint& a) {
		(x *= a.x) %= MOD;
		return *this;
	}
	mint operator+(const mint& a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint& a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint& a) const {
		mint res(*this);
		return res *= a;
	}

	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	mint operator^(const ll& t)const {
		if (!t) return 1;
		mint a = *this^(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime MOD
	mint inv() const {
		return pow(MOD - 2);
	}
	mint& operator/=(const mint& a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint& a) const {
		mint res(*this);
		return res /= a;
	}

	friend ostream& operator<<(ostream& os, const mint& m) {
		os << m.x;
		return os;
	}
};

///////////////////////////

ll N;
vector<vector<ll>> g;
vector<ll> A;

vector<ll> token;

void tokendfs(ll u, ll v) {
	token[v] += A[v];

	for (ll w : g[v]) {
		if (u == w) { continue; }
		tokendfs(v, w);
		token[v] += token[w];
	}

	return;
}

pair<ll, ll> dfs(ll u, ll v) {
	//cout << u << " " << v << endl<<flush;
	vector<ll> mx;
	vector<ll> mn;

	for (ll w : g[v]) {
		if (u == w) { continue; }
		auto p = dfs(v, w);

		mx.push_back(p.first + token[w]);
		mn.push_back(p.second + token[w]);
	}

	ll M = mx.size();
	//cout << M << endl << flush;
	ll fst = 0;
	rep(i, M) fst += mx[i];

	ll snd = 0;
	rep(i, M) {
		snd = max(snd, -fst + mx[i] + mn[i]);
	}

	if (snd == 0) { if (fst % 2 == 1) { snd = 1; } }

	return { fst,snd };
}


int main() {
	cin >> N;
	A.assign(N + 1, 0);

	string S;
	cin >> S;
	repn(i, N) { A[i] = S[i - 1] - '0'; }
	
	g.resize(N + 1);
	repn(i, N - 1) {
		ll a, b;
		cin >> a >> b;
		g[a].push_back(b);
		g[b].push_back(a);
	}


	ll ans = INF;

	repn(v, N) {
		token.assign(N + 1, 0);
		tokendfs(0, v);

		//repn(w, N) cout << token[w] <<endl<< flush;

		auto p = dfs(0, v);

		if (p.second == 0) { ans = min(ans, p.first / 2); }
	}

	if (ans == INF) { ans = -1; }
	cout << ans << endl;
}


Measured memory_bytes: 640