#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <sstream>
#include <complex>
#include <ctime>
#include <cassert>
#include <functional>

using namespace std;

typedef long long ll;
typedef vector<int> VI;
typedef pair<int,int> PII;

#define REP(i,s,t) for(int i=(s);i<(t);i++)
#define FILL(x,v) memset(x,v,sizeof(x))
#define MAXN 2505

VI adj[MAXN];
int ch[MAXN];
vector<VI> child;

void dfs(int x, int p, int t) {
  int c = ch[x];
  if (child[t][c] == -1) {
    child[t][c] = child.size();
    child.push_back(VI(16, -1));
  }
  for (int y : adj[x]) {
    if (y == p) continue;
    dfs(y, x, child[t][c]);
  }
}

int main() {
  int n;
  string hex;
  cin >> n >> hex;
  REP(i,0,n) ch[i] = (hex[i] >= '0' && hex[i] <= '9') ? (hex[i] - '0') : (hex[i] - 'a' + 10);
  REP(i,0,n-1) {
    int x, y;
    cin >> x >> y; x--; y--;
    adj[x].push_back(y);
    adj[y].push_back(x);
  }
  child.push_back(VI(16, -1));
  REP(i,0,n) dfs(i, -1, 0);
  cout << (int)child.size() - 1 << endl;
  return 0;
}