#include<bits/stdc++.h>
using namespace std;
void *wmem;
inline void rd(int &x){
  int k, m=0;
  x=0;
  for(;;){
    k = getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
inline int rd(char c[]){
  int i, sz=0;
  for(;;){
    i = getchar_unlocked();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c[sz++] = i;
  for(;;){
    i = getchar_unlocked();
    if(i==' '||i=='\n'||i=='\r'||i=='\t'||i==EOF){
      break;
    }
    c[sz++] = i;
  }
  c[sz]='\0';
  return sz;
}
inline void wt_L(char a){
  putchar_unlocked(a);
}
inline void wt_L(int x){
  char f[10];
  int m=0, s=0;
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    putchar_unlocked('-');
  }
  while(s--){
    putchar_unlocked(f[s]+'0');
  }
}
template<class S, class T> inline S chmin(S &a, T b){
  if(a>b){
    a=b;
  }
  return a;
}
template<class S, class T> inline S chmax(S &a, T b){
  if(a<b){
    a=b;
  }
  return a;
}
struct graph{
  int N, **edge, *es;
  void setEdge(int N__, int M, int A[], int B[], void **mem = &wmem){
    int i;
    N = N__;
    es = (int*)(*mem);
    edge = (int**)(es+N);
    edge[0] = (int*)(edge+N);
    for(i=0;i<N;i++){
      es[i] = 0;
    }
    for(i=0;i<M;i++){
      es[A[i]]++;
      es[B[i]]++;
    }
    for(i=1;i<N;i++){
      edge[i] = edge[i-1] + es[i-1];
    }
    (*mem) = edge[N-1] + es[N-1];
    for(i=0;i<N;i++){
      es[i] = 0;
    }
    for(i=0;i<M;i++){
      edge[A[i]][es[A[i]]++] = B[i];
      edge[B[i]][es[B[i]]++] = A[i];
    }
  }
  void setDirectEdge(int N__, int M, int A[], int B[], void **mem = &wmem){
    int i;
    N = N__;
    es = (int*)(*mem);
    edge = (int**)(es+N);
    edge[0] = (int*)(edge+N);
    for(i=0;i<N;i++){
      es[i] = 0;
    }
    for(i=0;i<M;i++){
      es[A[i]]++;
    }
    for(i=1;i<N;i++){
      edge[i] = edge[i-1] + es[i-1];
    }
    (*mem) = edge[N-1] + es[N-1];
    for(i=0;i<N;i++){
      es[i] = 0;
    }
    for(i=0;i<M;i++){
      edge[A[i]][es[A[i]]++] = B[i];
    }
  }
  graph reverse(void **mem = &wmem){
    graph g;
    int i, j, k;
    g.N = N;
    g.es = (int*)(*mem);
    g.edge = (int**)(g.es + N);
    g.edge[0] = (int*)(g.edge + N);
    for(i=0;i<N;i++){
      g.es[i] = 0;
    }
    for(i=0;i<N;i++){
      for(j=0;j<es[i];j++){
        g.es[edge[i][j]]++;
      }
    }
    for(i=1;i<N;i++){
      g.edge[i] = g.edge[i-1] + g.es[i-1];
    }
    *mem = g.edge[N-1] + g.es[N-1];
    for(i=0;i<N;i++){
      g.es[i] = 0;
    }
    for(i=0;i<N;i++){
      for(j=0;j<es[i];j++){
        k = edge[i][j];
        g.edge[k][g.es[k]++] = i;
      }
    }
    return g;
  }
  graph reduce(int tn, int ind[], int self_e = 0, int dep_e = 0, void **mem = &wmem){
    graph g;
    int M=0, i, j, k, x, y;
    pair<int,int> *A;
    for(i=0;i<N;i++){
      M += es[i];
    }
    A = (pair<int,int>*)((int*)((int**)(*mem) + tn) + tn + M);
    M = 0;
    for(i=0;i<N;i++){
      x = ind[i];
      if(x < 0){
        continue;
      }
      for(j=0;j<es[i];j++){
        y = ind[edge[i][j]];
        if(y < 0){
          continue;
        }
        if(self_e==0 && x==y){
          continue;
        }
        A[M++] = make_pair(x, y);
      }
    }
    if(dep_e==0){
      sort(A, A+M);
      k = 0;
      for(i=0;i<M;i++){
        if(k && A[k-1]==A[i]){
          continue;
        }
        A[k++] = A[i];
      }
      M = k;
    }
    g.N = tn;
    g.es = (int*)(*mem);
    g.edge = (int**)(g.es + tn);
    g.edge[0] = (int*)(g.edge + tn);
    for(i=0;i<tn;i++){
      g.es[i] = 0;
    }
    for(i=0;i<M;i++){
      g.es[A[i].first]++;
    }
    for(i=1;i<tn;i++){
      g.edge[i] = g.edge[i-1] + g.es[i-1];
    }
    *mem = g.edge[tn-1] + g.es[tn-1];
    for(i=0;i<tn;i++){
      g.es[i] = 0;
    }
    for(i=0;i<M;i++){
      j = A[i].first;
      k = A[i].second;
      g.edge[j][g.es[j]++] = k;
    }
    return g;
  }
  void getDist(int root, int res[], void *mem = wmem){
    int i, j, k, *q, s, z;
    q=(int*)mem;
    for(i=0;i<N;i++){
      res[i]=-1;
    }
    res[root]=0;
    s=0;
    z=1;
    q[0]=root;
    while(z){
      i=q[s++];
      z--;
      for(j=0;j<es[i];j++){
        k=edge[i][j];
        if(res[k]>=0){
          continue;
        }
        res[k]=res[i]+1;
        q[s+z++]=k;
      }
    }
  }
  inline int sccDFS(int num[], int st, int mx){
    int i, j;
    num[st]=-2;
    for(i=0;i<es[st];i++){
      j=edge[st][i];
      if(num[j]==-1){
        mx=sccDFS(num,j,mx);
      }
    }
    num[st]=mx;
    return mx+1;
  }
  int scc(int res[], void *mem = wmem){
    graph r;
    int i, j, k, *nrv, *num, ret=0, *st, st_size;
    r = reverse(&mem);
    st = (int*)mem;
    num = st+N;
    nrv = num + N;
    for(i=0;i<N;i++){
      res[i] = num[i] = -1;
    }
    k = 0;
    for(i=0;i<N;i++){
      if(num[i]==-1){
        k = sccDFS(num,i,k);
      }
    }
    for(i=0;i<N;i++){
      nrv[num[i]] = i;
    }
    for(k=N-1;k>=0;k--){
      i=nrv[k];
      if(res[i]>=0){
        continue;
      }
      res[i]=ret;
      st_size=0;
      st[st_size++]=i;
      while(st_size){
        i=st[--st_size];
        for(j=0;j<r.es[i];j++){
          if(res[r.edge[i][j]]==-1){
            res[r.edge[i][j]]=ret;
            st[st_size++]=r.edge[i][j];
          }
        }
      }
      ret++;
    }
    return ret;
  }
  inline void bccDFS(int v, int u, int *res, int *rt, int &rts, int *S, int &Ss, int *inS, int *num, int &tm){
    int i, k;
    num[v] = ++tm;
    S[Ss++] = v;
    inS[v] = 1;
    rt[rts++] = v;
    for(i=0;i<es[v];i++){
      int w=edge[v][i];
      if(!num[w]){
        bccDFS(w, v, res, rt, rts, S, Ss, inS, num, tm);
      }
      else if(u != w && inS[w]){
        while(num[rt[rts-1]] > num[w]){
          rts--;
        }
      }
    }
    if(v == rt[rts-1]){
      k = S[Ss-1];
      for(;;){
        int w=S[--Ss];
        inS[w] = 0;
        res[w] = k;
        if(v==w){
          break;
        }
      }
      rts--;
    }
  }
  int bcc(int res[], void *mem=wmem){
    int *S, Ss=0, i, *inS, k, *num, *rt, rts=0, tm=0;
    pair<int,int> *arr;
    num = (int*)mem;
    rt = num + N;
    S = rt + N;
    inS = S + N;
    memset(num, 0, sizeof(int)*N);
    memset(inS, 0, sizeof(int)*N);
    for(i=0;i<N;i++){
      if(!num[i]){
        bccDFS(i, N, res, rt, rts, S, Ss, inS, num, tm);
      }
    }
    arr = (pair<int,int>*)mem;
    for(i=0;i<N;i++){
      arr[i].first = res[i];
      arr[i].second = i;
    }
    sort(arr, arr+N);
    k = 0;
    for(i=0;i<N;i++){
      if(i && arr[i].first != arr[i-1].first){
        k++;
      }
      res[arr[i].second] = k;
    }
    return k+1;
  }
}
;
char memarr[96000000];
int N;
char S[2100];
int A[2100];
int B[2100];
graph g;
int res;
int s[2000];
int sz;
int t[2000];
int tz;
pair<int,pair<int,int> > dfs(int n, int b, int d){
  int i, j, mn, sm, tp;
  pair<int,pair<int,int> > res, tmp;
  vector< pair<int,int> > s;
  res.first = 0;
  res.second.first = 0;
  for(i=0;i<g.es[n];i++){
    if(g.edge[n][i] != b){
      tmp = dfs(g.edge[n][i], n, d+1);
      res.first += tmp.first;
      res.second.first += tmp.second.first + tmp.first;
      s.push_back( make_pair(tmp.second.first + tmp.first, tmp.second.second + tmp.first) );
    }
  }
  sm = 0;
  for(j=0;j<s.size();j++){
    sm += s[j].first;
  }
  mn = 0;
  for(i=0;i<s.size();i++){
    tp = s[i].second - sm + s[i].first;
    if(tp < 0){
      tp = (tp + 1000000000) % 2;
    }
    chmax(mn, tp);
  }
  res.second.second = mn;
  if(S[n]=='1'){
    res.first++;
  }
  return res;
}
int main(){
  int i, j, k, sm, x, y, z;
  pair<int,pair<int,int> > r;
  wmem = memarr;
  rd(N);
  rd(S);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=0;Lj4PdHRW<N-1;Lj4PdHRW++){
      rd(A[Lj4PdHRW]);
      rd(B[Lj4PdHRW]);
    }
  }
  for(i=0;i<N-1;i++){
    A[i]--;
  }
  for(i=0;i<N-1;i++){
    B[i]--;
  }
  g.setEdge(N, N-1, A, B);
  res = 1073709056;
  for(i=0;i<N;i++){
    sz = tz = 0;
    for(j=0;j<g.es[i];j++){
      r = dfs(g.edge[i][j], i, 1);
      s[sz++] = r.second.first + r.first;
      t[tz++] = r.second.second + r.first;
    }
    {
      int KL2GvlyY, Q5VJL1cS;
      if(sz==0){
        Q5VJL1cS = 0;
      }
      else{
        Q5VJL1cS = s[0];
        for(KL2GvlyY=1;KL2GvlyY<sz;KL2GvlyY++){
          Q5VJL1cS += s[KL2GvlyY];
        }
      }
      k =Q5VJL1cS;
    }
    if(k%2){
      continue;
    }
    {
      int cTE1_r3A, e98WHCEY;
      if(sz==0){
        cTE1_r3A = 0;
      }
      else{
        cTE1_r3A = s[0];
        for(e98WHCEY=1;e98WHCEY<sz;e98WHCEY++){
          cTE1_r3A += s[e98WHCEY];
        }
      }
      sm =cTE1_r3A;
    }
    for(x=0;x<sz;x++){
      z = t[x] - sm + s[x];
      if(z > 0){
        break;
      }
    }
    if(x!=sz){
      continue;
    }
    chmin(res, k/2);
  }
  if(res==1073709056){
    res = -1;
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay varsion 20190601-1

// --- original code ---
// int N;
// char S[2100];
// int A[2100], B[2100];
// 
// graph g;
// int res;
// 
// int s[2000], sz;
// int t[2000], tz;
// 
// pair<int,pair<int,int> > dfs(int n, int b, int d){
//   int i, j, mn, tp, sm;
//   pair<int,pair<int,int> > res, tmp;
//   res.first = 0;
//   res.second.first = 0;
// 
//   vector< pair<int,int> > s;
// 
//   rep(i,g.es[n]){
//     if(g.edge[n][i] != b){
//       tmp = dfs(g.edge[n][i], n, d+1);
//       res.first += tmp.first;
//       res.second.first += tmp.second.first + tmp.first;
//       s.push_back( make_pair(tmp.second.first + tmp.first, tmp.second.second + tmp.first) );
//     }
//   }
// 
//   sm = 0;
//   rep(j,s.size()) sm += s[j].first;
//   mn = 0;
//   rep(i,s.size()){
//     tp = s[i].second - sm + s[i].first;
//     if(tp < 0) tp = (tp + 1d9) % 2;
//     mn >?= tp;
//   }
// 
//   res.second.second = mn;
//   if(S[n]=='1') res.first++;
//   
//   return res;
// }
// 
// {
//   int i, j, k, x, y, z, sm;
//   pair<int,pair<int,int> > r;
//   
//   rd(N,S,(A,B)(N-1));
//   rep(i,N-1) A[i]--;
//   rep(i,N-1) B[i]--;
//   g.setEdge(N, N-1, A, B);
// 
//   res = int_inf;
// 
//   rep(i,N){
//     sz = tz = 0;
//     rep(j,g.es[i]){
//       r = dfs(g.edge[i][j], i, 1);
//       s[sz++] = r.second.first + r.first;
//       t[tz++] = r.second.second + r.first;
//     }
//     k = sum(s(sz));
//     if(k%2) continue;
// 
//     sm = sum(s(sz));
//     rep(x,sz){
//       z = t[x] - sm + s[x];
//       if(z > 0) break;
//     }
//     if(x!=sz) continue;
//     res <?= k/2;
//   }
// 
//   if(res==int_inf) res = -1;
//   wt(res);
// }


Measured memory_bytes: 640