/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author majk
 */

#ifndef MAJK_LIB
#define MAJK_LIB

#include <vector>
#include <stack>
#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <iomanip>
#include <set>
#include <functional>
#include <fstream>
#include <algorithm>
#include <cassert>
#include <cmath>
#include <string>
#include <sstream>
#include <queue>
#include <array>
#include <bitset>
using namespace std;

#define x first
#define y second
typedef std::pair<int,int> pii; typedef long long ll; typedef unsigned long long ull; typedef unsigned int ui; typedef pair<ui,ui> puu;

template <typename T, typename U> std::istream&operator>>(std::istream&i, pair<T,U>&p) {i >> p.x >> p.y; return i;}
template<typename T>std::istream&operator>>(std::istream&i,vector<T>&t) {for(auto&v:t){i>>v;}return i;}
template <typename T, typename U> std::ostream&operator<<(std::ostream&o, const pair<T,U>&p) {o << p.x << ' ' << p.y; return o;}
template<typename T>std::ostream&operator<<(std::ostream&o,const vector<T>&t) {if(t.empty())o<<'\n';for(size_t i=0;i<t.size();++i){o<<t[i]<<" \n"[i == t.size()-1];}return o;}
template<typename T> using minheap = priority_queue<T, vector<T>, greater<T>>;
template<typename T> using maxheap = priority_queue<T, vector<T>, less<T>>;
template <typename T> bool in(T a, T b, T c) { return a <= b && b < c; }
ui logceil(ll x) { return x?8*sizeof(ll)-__builtin_clzll(x):0; }

namespace std { template<typename T,typename U>struct hash<pair<T,U>>{hash<T>t;hash<U>u;size_t operator()(const pair<T,U>&p)const{return t(p.x)^(u(p.y)<<7);}}; }
template<typename T,typename F>T bsh(T l,T h,const F&f){T r=-1,m;while(l<=h){m=(l+h)/2;if(f(m)){l=m+1;r=m;}else{h=m-1;}}return r;}
template<typename F> double bshd(double l,double h,const F&f,double p=1e-9){ui r=3+(ui)log2((h-l)/p);while(r--){double m=(l+h)/2;if(f(m)){l=m;}else{h=m;}}return (l+h)/2;}
template<typename T,typename F>T bsl(T l,T h,const F&f){T r=-1,m;while(l<=h){m=(l+h)/2;if(f(m)){h=m-1;r=m;}else{l=m+1;}}return r;}
template<typename F> double bsld(double l,double h,const F&f,double p=1e-9){ui r=3+(ui)log2((h-l)/p);while(r--){double m=(l+h)/2;if(f(m)){h=m;}else{l=m;}}return (l+h)/2;}
template<typename T> T gcd(T a,T b) { if (a<b) swap(a,b); return b?gcd(b,a%b):a; }


template<typename T>class vector2:public vector<vector<T>>{public:vector2(){} vector2(size_t a,size_t b,T t=T()):vector<vector<T>>(a,vector<T>(b,t)){}};
template<typename T>class vector3:public vector<vector2<T>>{public:vector3(){} vector3(size_t a,size_t b,size_t c,T t=T()):vector<vector2<T>>(a,vector2<T>(b,c,t)){}};
template<typename T>class vector4:public vector<vector3<T>>{public:vector4(){} vector4(size_t a,size_t b,size_t c,size_t d,T t=T()):vector<vector3<T>>(a,vector3<T>(b,c,d,t)){}};
template<typename T>class vector5:public vector<vector4<T>>{public:vector5(){} vector5(size_t a,size_t b,size_t c,size_t d,size_t e,T t=T()):vector<vector4<T>>(a,vector4<T>(b,c,d,e,t)){}};


#endif

class ECompleteCompress {
public:
    string S;
    int N;
    vector<vector<int>> E;
    vector<int> SZ, P;
    vector<ll> D;
    vector<ll> UP;
    ll ans;
//
//    void dfs(int u, int p) {
//        P[u] = p;
//        SZ[u] = S[u] == '1';
//        D[u] = 0;
//        for (int v : E[u]) {
//            if (v != p) {
//                dfs(v, u);
//                SZ[u] += SZ[v];
//                D[u] += D[v];
//                D[u] += SZ[v];
//            }
//        }
//    }
//
//    void dfs2(int u, int cnt, ll depth) {
//        UP[u] = depth;
//        ll tot = 0;
//        for (int v : E[u]) if (v != P[u]) {
//            tot += D[v] + SZ[v];
//            cnt += SZ[v];
//        }
//
//        for (int v : E[u]) {
//            if (v != P[u]) {
//                ll mine = tot + cnt - (2*SZ[v] + D[v]);
//                dfs2(v, cnt - SZ[v], mine);
//            }
//        }
//    }

    int cnt(int u, int p, int d) {
        int ans = 0;
        if (S[u] == '1') ans += d;
        for (int v : E[u]) if (v != p) {
            ans += cnt(v, u, d+1);
        }
        return ans;
    }

    pair<int, pii> solve(int u, int p) {
        vector<pair<int, pii>> G;
        for (int v : E[u]) {
            if (v != p) {
                G.push_back(solve(v, u));
            }
        }

        int size = 0;
        int maxDepth = 0;
        int minDepth = 0;
        for (auto g: G) {
            maxDepth += g.y.y;
            size += g.x;
            minDepth += g.y.x;
        }

        int cur = maxDepth % 2;
        for (auto g: G) {
            int bst = max(0, g.y.x - (maxDepth - g.y.y));
            cur = max(cur, bst);
        }

        if (S[u] == '1') ++size;
//        cerr << u << ' ' << size << ' ' << cur + size << ' ' << maxDepth + size << endl;
        return {size, {cur + size, maxDepth + size}};

    }


    void solve(istream& cin, ostream& cout) {
        cin >> N >> S;
        E.resize(N);
        for (int i = 0; i < N - 1; ++i) {
            int u, v;
            cin >> u >> v;
            --u;
            --v;
            E[u].push_back(v);
            E[v].push_back(u);
        }

        int ans = 1000000000;
        for (int i = 0; i < N; ++i) {
            auto a = solve(i, -1);
//            cout << a << endl;
            if (a.y.x == a.x) {
                ans = min(ans, (a.y.y-a.y.x)/2);
            }
//            vector<ll> Q;
//            for (int v : E[i]) {
//                Q.push_back(cnt(v, i, 1));
////                if (v == P[i]) {
////                    Q.push_back(UP[i]);
////                } else {
////                    Q.push_back(D[v] + SZ[v]);
////                }
//            }
//            sort(Q.begin(),Q.end());
////            cout << Q << endl;
//            ll tot = 0;
//            for (ll q: Q) tot += q;
//            if (tot % 2 == 0 && Q.back() <= tot/2) {
//                ans = min(ans, tot/2);
//            }
        }
        if (ans < 1000000000) {
            cout << ans << endl;
        } else {
            cout << -1 << endl;
        }
    }
};


int main() {
	ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
	ECompleteCompress solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
    return 0;
}


Measured memory_bytes: 512