from collections import defaultdict as ddict

import sys
sys.setrecursionlimit(100000)



def dfs(now, pre):
    ret = [0, 0, 0]

    for nxt in G[now]:
        if nxt == pre:
            continue
        child = dfs(nxt, now)
        for i in range(3):
            ret[i] += child[i]

    if s[now] == 'c':
        ret[2] += ret[1]
    else:
        ret[1] += ret[0]
        ret[0] += 1

    return ret

#################################

N = int(input())
s = input()
G = ddict(list)

for i in range(N - 1):
    a, b = list(map(int, input().split()))
    a -= 1; b -= 1
    G[a].append(b)
    G[b].append(a)

ans = 0
for i in range(N):
    if s[i] == 'c':
        ans += dfs(i, -1)[2]

print(ans)