from collections import defaultdict as ddict
import sys
sys.setrecursionlimit(1000000)

def dfs(v, par):
    ret = [0, 0]    # (ww, w)
    for ch in G[v]:
        if ch == par:
            continue
        child = dfs(ch, v)
        ret[0] += child[0]
        ret[1] += child[1]

    if s[v] == 'w':
        ret[0] += ret[1]
        ret[1] += 1

    C[v] = ret
    return ret

ans = 0
def dfs2(v, par, p):
    global ans
    if s[v] == 'c':
        ans += p[0]
        for ch in G[v]:
            if ch == par:
                continue
            ans += C[ch][0]

    for ch in G[v]:
        if ch == par:
            continue
        tmp = list(p)
        tmp[0] += C[v][0]; tmp[1] += C[v][1]
        tmp[0] -= C[ch][0]; tmp[1] -= C[ch][1]
        if s[v] == 'w':
            tmp[0] -= C[ch][1]
            tmp[0] += p[1]
        dfs2(ch, v, tmp)

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

C = [None] * N
dfs(0, -1)
dfs2(0, -1, [0, 0])
print(ans)