#pragma GCC optimize("Ofast,unroll-loops,no-stack-protector,fast-math")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma comment(linker, "/stack:200000000")

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<ld,ld> pdd;

#define X first
#define Y second

//#include <boost/unordered_map.hpp>
//using namespace boost;

/*
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> rbtree;
rbtree T;
*/

namespace io{
	const int L = (1 << 20) + 1;

	char buf[L], *S , *T, c;

	char getchar() {
		if(__builtin_expect(S == T, 0)) {
			T = (S = buf) + fread(buf, 1, L, stdin);
			return (S == T ? EOF : *S++);
		}
		return *S++;
	}

	int inp() {
		int x = 0, f = 1; char ch;
		for(ch = getchar(); !isdigit(ch); ch = getchar())
			if(ch == '-') f = -1;
		for(; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		return x * f;
	}

	unsigned inpu()
	{
		unsigned x = 0; char ch;
		for(ch = getchar(); !isdigit(ch); ch = getchar());
		for(; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		return x;
	}

	ll inp_ll() {
		ll x = 0; int f = 1; char ch;
		for(ch = getchar(); !isdigit(ch); ch = getchar())
			if(ch == '-') f = -1;
		for(; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		return x * f;
	}

	char B[45], *outs=B+40, *outr=B+40;
	template<class T>
	inline void print(register T a,register char x=0){
		if(x) *--outs = x, x = 0;

		if(!a)*--outs = '0';
		else
			while(a)
				*--outs = (a % 10) + 48, a /= 10;

		if(x)
			*--outs = x;

		fwrite(outs, outr - outs , 1, stdout);
		outs = outr;
	}
};

using io :: print;
using io :: inp;
using io :: inpu;
using io :: inp_ll;

using i32 = int;
using i64 = long long;
using u8 = unsigned char;
using u32 = unsigned;
using u64 = unsigned long long;
using f64 = double;
using f80 = long double;
//using i128 = __int128_t;
//using u128 = __uint128_t;
using i128 = i64;
using u128 = u64;

ll power(ll a, ll b, ll p)
{
	if (!b) return 1;
	ll t = power(a, b/2, p);
	t = t*t%p;
	if (b&1) t = t*a%p;
	return t;
}

ll exgcd(ll a, ll b, ll &x, ll &y)
{
	if (b == 0)
	{
		x = 1;
		y = 0;
		return a;
	}
	ll px, py;
	ll d = exgcd(b, a%b, px, py);
	x = py;
	y = px-a/b*py;
	return d;
}

template<class T>
inline void freshmin(T &a, const T &b)
{
	if (a > b) a = b;
}

template<class T>
inline void freshmax(T &a, const T &b)
{
	if (a < b) a = b;
}

const int MOD = 1000003;
const int INF = 1000000000;
const int MAXN = 2010;
const int MAXK = 18;

int n;
char s[MAXN];
vector<int> v[MAXN];

int F[MAXN], dis[MAXN], size[MAXN], tot[MAXN];
int ret;

void dfs(int x, int p)
{
	size[x] = s[x] == '1';
	tot[x] = 0;
	vector<pii> Q;
	for (auto y : v[x])
	{
		if (y == p) continue;
		dis[y] = dis[x]+1;
		dfs(y, x);
		size[x] += size[y];
		tot[x] += tot[y]+size[y];
		Q.push_back({F[y]+size[y], tot[y]+size[y]});
	}
	if (Q.size() >= 2)
	{
		sort(Q.begin(), Q.end());
		reverse(Q.begin(), Q.end());
		int sum = 0;
		for (int i = 1; i < Q.size(); ++ i)
			sum += Q[i].X;
		if (Q[0].X > sum)
		{
			int tmp = Q[0].X-sum;
			for (int i = 1; i < Q.size(); ++ i)
			{
				tmp -= Q[i].Y-Q[i].X;
				if (tmp < 0) tmp = 0;
			}
			F[x] = tmp;
		}
		else
		{
			if ((Q[0].X+sum)%2 == 0)
				F[x] = 0;
			else
			{
				F[x] = 1;
			}
		}
	}
	else if (Q.size() == 1)
		F[x] = Q[0].X;
	else
		F[x] = 0;
}

int main()
{
	
	scanf("%d%s", &n, s+1);
	for (int i = 1; i < n; ++ i)
	{
		int x, y;
		scanf("%d%d", &x, &y);
		v[x].push_back(y);
		v[y].push_back(x);
	}
	
	int ans = INF;
	
	for (int i = 1; i <= n; ++ i)
	{
		dis[i] = 0;
		ret = 0;
		dfs(i, 0);
		if (F[i] == 0 && tot[i]%2 == 0)
			ans = min(ans, tot[i]/2);
	}
	
	if (ans == INF) ans = -1;
	cout << ans << endl;
	
	return 0;
}


Measured memory_bytes: 640