#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[2002];
int A[2000];
int B[2000];
graph g;
int cnt[2000];
int mn[2000];
int mx[2000];
void dfs(int n, int b){
  int i, k, t;
  for(i=0;i<g.es[n];i++){
    k = g.edge[n][i];
    if(k==b){
      continue;
    }
    dfs(k, n);
  }
  cnt[n] = mx[n] = 0;
  for(i=0;i<g.es[n];i++){
    k = g.edge[n][i];
    if(k==b){
      continue;
    }
    cnt[n] += cnt[k];
    mx[n] += mx[k] + cnt[k];
  }
  mn[n] = 0;
  for(i=0;i<g.es[n];i++){
    k = g.edge[n][i];
    if(k==b){
      continue;
    }
    t = (mn[k]+cnt[k]) - mx[n] + (mx[k]+cnt[k]);
    if(t < 0){
      t = (t + 1000000000) % 2;
    }
    chmax(mn[n], t);
  }
  if(S[n]=='1'){
    cnt[n]++;
  }
}
int main(){
  int i, j, k, n, res, s;
  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(n=0;n<N;n++){
    s = 0;
    for(i=0;i<g.es[n];i++){
      k = g.edge[n][i];
      dfs(k, n);
      s += mx[k] + cnt[k];
    }
    if(s%2){
      continue;
    }
    for(i=0;i<g.es[n];i++){
      k = g.edge[n][i];
      j = (mn[k]+cnt[k]) - s + (mx[k]+cnt[k]);
      if(j > 0){
        break;
      }
    }
    if(i != g.es[n]){
      continue;
    }
    chmin(res, s/2);
  }
  if(res==1073709056){
    res = -1;
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay varsion 20190601-1

// --- original code ---
// int N;
// char S[2002];
// int A[2000], B[2000];
// 
// graph g;
// 
// int cnt[2000], mn[2000], mx[2000];
// void dfs(int n, int b){
//   int i, k, t;
// 
//   rep(i, g.es[n]){
//     k = g.edge[n][i];
//     if(k==b) continue;
//     dfs(k, n);
//   }
// 
//   cnt[n] = mx[n] = 0;
//   rep(i, g.es[n]){
//     k = g.edge[n][i];
//     if(k==b) continue;
//     cnt[n] += cnt[k];
//     mx[n] += mx[k] + cnt[k];
//   }
// 
//   mn[n] = 0;
//   rep(i, g.es[n]){
//     k = g.edge[n][i];
//     if(k==b) continue;
//     t = (mn[k]+cnt[k]) - mx[n] + (mx[k]+cnt[k]);
//     if(t < 0) t = (t + 1d9) % 2;
//     mn[n] >?= t;
//   }
// 
//   if(S[n]=='1') cnt[n]++;
// }
// 
// {
//   int i, j, k, n, s;
//   int res;
//   
//   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(n,N){
//     s = 0;
//     rep(i, g.es[n]){
//       k = g.edge[n][i];
//       dfs(k, n);
//       s += mx[k] + cnt[k];
//     }
//     if(s%2) continue;
// 
//     rep(i, g.es[n]){
//       k = g.edge[n][i];
//       j = (mn[k]+cnt[k]) - s + (mx[k]+cnt[k]);
//       if(j > 0) break;
//     }
//     if(i != g.es[n]) continue;
// 
//     res <?= s/2;
//   }
// 
//   if(res==int_inf) res = -1;
//   wt(res);
// }


Measured memory_bytes: 512