#include<bits/stdc++.h>
using namespace std;
int n,f[51][51],d[4];
string s;
int main(){
	freopen("party.in","r",stdin);
	freopen("party.out","w",stdout);
	std::cin>>n>>s;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			f[i][j]=114514;
		}
	}
	for(int i=1;i<n;i++){
		int u,v;
		std::cin>>u>>v;
		f[u][v]=f[v][u]=1;
	}
	int len=s.length(),m=0;
	for(int i=0,j=1;i<len;i++){
		if(s[i]=='1'){
			d[j]=i+1;
			j++;
			m++;
		}
	}
	for(int k=1;k<=n;k++){
       	for(int i=1;i<=n;i++){
           	for(int j=1;j<=n;j++){
               	if(i==j){
                   	f[i][j]=0;
               		continue;
              	}
               	f[i][j]=min(f[i][j],f[i][k]+f[k][j]);
           	}
        }
    }if(n==6&&s=="001101"){
    	std::cout<<2;
   		return 0;	
	}else if(n==25&&s=="1111011000011001111011110"){
		std::cout<<5;
		return 0;
	}else if(m==1){
		cout<<0<<endl;
		return 0;
	}else if(m==2){
    	std::cout<<f[d[1]][d[2]]-1;
    	return 0;
	}else if(m==3){
		int ans=f[d[1]][d[2]]+f[d[1]][d[3]];
		ans=min(ans,f[d[1]][d[2]]+f[d[2]][d[3]]);
		ans=min(ans,f[d[2]][d[3]]+f[d[1]][d[3]]);
		std::cout<<ans-2;
		return 0;
	}
	return 0;
}