#include <bits/stdc++.h>
using namespace std;
#define rep(i, n)  for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define all(a) a.begin(),a.end()
#define pb emplace_back
#define eb emplace_back
#define lb(v,k) (lower_bound(all(v),k)-v.begin())
#define ub(v,k) (upper_bound(all(v),k)-v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T,vector<T>,greater<T>>
#define dame(a) {out(a);return 0;}
#define decimal cout<<fixed<<setprecision(15);
#define dupli(a) erase(unique(all(a)),a.end())
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
typedef multiset<ll> S;
using vi=vector<ll>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vp=vector<P>;
using vvp=vector<vp>;
using vb=vector<bool>;
using vvb=vector<vb>;
const ll inf=1001001001001001001;
const int INF=1001001001;
const int mod=1000000007;
const double eps=1e-10;
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yesno(T b){if(b)out("yes");else out("no");}
template<class T> void YesNo(T b){if(b)out("Yes");else out("No");}
template<class T> void YESNO(T b){if(b)out("YES");else out("NO");}
template<class T> void noyes(T b){if(b)out("no");else out("yes");}
template<class T> void NoYes(T b){if(b)out("No");else out("Yes");}
template<class T> void NOYES(T b){if(b)out("NO");else out("YES");}
void outs(ll a,ll b){if(a>=inf-100)out(b);else out(a);}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){a%=mod;if(b==0)return 1;if(b&1)return a*modpow(a,b-1)%mod;ll k=modpow(a,b/2);return k*k%mod;}
vi dep,par,sz,num;
vvi g;
ll dfs0(int i,int p,int d){
    par[i]=p;
    dep[i]=d;
    sz[i]=num[i];
    for(ll x:g[i])if(x!=p)sz[i]+=dfs0(x,i,d+1);
    return sz[i];
}
void tree(ll n,ll s){
    dep=vi(n);
    par=vi(n);
    sz=vi(n);
    dfs0(s,-1,0);
}
vp dp;
P dfs(int i){
    ll mi,ma=0;
    P m=P(-inf,inf);
    for(ll x:g[i])if(x!=par[i]){
        auto p=dfs(x);
        ma+=p.se+sz[x];
        chmax(m,P(p.fi+sz[x],p.se+sz[x]));
    }
    mi=max(0ll,m.fi-(ma-m.se));
    if(mi==0)mi+=ma%2;
    return P(mi,ma);
}
int main(){
    ll n,ans=inf;cin>>n;
    string str;cin>>str;
    num=vi(n);
    g=vvi(n);
    rep(i,n-1){
        ll a,b;cin>>a>>b;a--;b--;
        g[a].pb(b);
        g[b].pb(a);
    }
    rep(i,n)if(str[i]=='1')num[i]++;
    rep(s,n){
        tree(n,s);
        P p=dfs(s);
        if(p.fi==0)chmin(ans,p.se);
    }
    if(ans==inf)out(-1);
    else outs(ans/2,-1);
}

Measured memory_bytes: 640