def main():
    n = int(input())
    tree = [[] for _ in range(n)]
    s = ['0123456789abcdef'.index(c) for c in input()]
    trie = [[0 for _ in range(16)]]
    def insert(trie_node, val):
        if trie[trie_node][val] == 0:
            nxt = len(trie)
            trie[trie_node][val] = nxt
            trie.append([0 for _ in range(16)])

        return trie[trie_node][val]

    for i in range(1, n):
        u, v = map(int, input().split())
        u -= 1
        v -= 1
        tree[u].append(v)
        tree[v].append(u)

    for root in range(n):
        stk = [(root, root, insert(0, s[root]))]
        while stk:
            u, p, t = stk.pop()
            for v in tree[u]:
                if v == p:
                    continue
                stk.append((v, u, insert(t, s[v])))

    # Don't count the empty string!
    print(len(trie) - 1)


if __name__ == '__main__':
    main()