#include<bits/stdc++.h>
#define R register
#define rep(i,a,b) for(R signed i=a ; i<=b ; ++i)
#define dwn(i,a,b) for(R signed i=a ; i>=b ; --i)
#define go(i,u) for(R signed i=head[u] ; i ; i=a[i].next)
#define min(a,b) (a<b?a:b)
#define max(a,b) (a>b?a:b)
using namespace std;

inline int read() {
	int res=0,f=1;char ch;
	while(!isdigit(ch=getchar())) if(ch=='-') f=-1;
	do {
		res=res*10+ch-'0';
	} while(isdigit(ch=getchar()));
	return res*f;
}

const int N=2005;

struct Edge {
	int next,to;
}a[N<<1];

int head[N],size;

inline void add(int u,int v) {
	a[++size].next=head[u],a[size].to=v,head[u]=size;
}

inline void insert(int u,int v) {
	add(u,v),add(v,u);
}

bool ok[N];

int n,dp[N],num[N],sum[N];

void dfs(int u,int fa) {
	num[u]=ok[u],sum[u]=0;
	int maxx=0,id=0;
	go(i,u) {
		int v=a[i].to;
		if(v!=fa) {
			dfs(v,u);
			num[u]+=num[v];
			sum[u]+=sum[v]+num[v];
			if(sum[v]+num[v]>maxx) maxx=sum[v]+num[v],id=v;
		}
	}
	if(!id) dp[u]=0;
	else if(sum[u]-maxx>=maxx) dp[u]=sum[u]/2;
	else dp[u]=sum[u]-maxx+min(dp[id],(2*maxx-sum[u])/2);
}

int main() {
	n=read();
	rep(i,1,n) scanf("%1d",&ok[i]);
	rep(i,2,n) insert(read(),read());
	int ans=0x3fffffff;
	rep(i,1,n) {
		dfs(i,0);
		if(sum[i]&1) continue ;
		if(dp[i]==sum[i]/2) ans=min(ans,dp[i]);
	}
	printf("%d",ans!=0x3fffffff?ans:-1);
	return 0;
}