import java.util.*;
import java.io.*;

public class GuidGeneratorMN {

  static class Kattio extends PrintWriter {
    public Kattio(InputStream i) {
      super(new BufferedOutputStream(System.out));
      this.r = new BufferedReader(new InputStreamReader(i));
    }

    public Kattio(InputStream i, OutputStream o) {
      super(new BufferedOutputStream(o));
      this.r = new BufferedReader(new InputStreamReader(i));
    }

    public boolean hasMoreTokens() {
      return this.peekToken() != null;
    }

    public int getInt() {
      return Integer.parseInt(this.nextToken());
    }

    public double getDouble() {
      return Double.parseDouble(this.nextToken());
    }

    public long getLong() {
      return Long.parseLong(this.nextToken());
    }

    public String getWord() {
      return this.nextToken();
    }

    private BufferedReader r;
    private String line;
    private StringTokenizer st;
    private String token;

    private String peekToken() {
      if (this.token == null) {
        try {
          while (this.st == null || !this.st.hasMoreTokens()) {
            this.line = this.r.readLine();
            if (this.line == null) {
              return null;
            }
            this.st = new StringTokenizer(this.line);
          }
          this.token = this.st.nextToken();
        } catch (IOException e) {
        }
      }

      return this.token;
    }

    private String nextToken() {
      String ans = this.peekToken();
      this.token = null;
      return ans;
    }
  }

  static class TrieNode {
    public int cnt;
    public TrieNode[] children;

    public TrieNode() {
      this.cnt = 0;
      this.children = new TrieNode[16];
      Arrays.fill(this.children, null);
    }
  }

  static int convert(char c) {
    if (Character.isDigit(c)) {
      return (int) (c) - 48;
    }
    return (int) (c) - 87;
  }

  static ArrayList<Integer> v = new ArrayList<>();
  static ArrayList<Integer>[] graph;

  static void dfs(int curr, int par, TrieNode node) {
    for (int neighbor : graph[curr]) {
      if (neighbor != par) {
        if (node.children[v.get(neighbor)] == null) {
          node.children[v.get(neighbor)] = new TrieNode();
        }
        dfs(neighbor, curr, node.children[v.get(neighbor)]);
      }
    }
  }

  static void trieDfs(TrieNode node) {
    node.cnt++;
    for (var child : node.children) {
      if (child != null) {
        trieDfs(child);
        node.cnt += child.cnt;
      }
    }
  }

  public static void main(String[] args) throws IOException {
    Kattio io = new Kattio(System.in, System.out);

    int n = io.getInt();
    String s = io.getWord();

    for (int i = 0; i < s.length(); i++) {
      v.add(convert(s.charAt(i)));
    }

    graph = new ArrayList[n];
    for (int i = 0; i < n; i++) {
      graph[i] = new ArrayList<>();
    }
    for (int i = 0; i < n - 1; i++) {
      int u = io.getInt(), v = io.getInt();
      graph[u - 1].add(v - 1);
      graph[v - 1].add(u - 1);
    }

    TrieNode root = new TrieNode();
    for (int i = 0; i < n; i++) {
      if (root.children[v.get(i)] == null) {
        root.children[v.get(i)] = new TrieNode();
      }
      dfs(i, -1, root.children[v.get(i)]);
    }

    trieDfs(root);

    io.println(root.cnt - 1);

    io.close();
  }
}