from functools import reduce
from math import factorial

N = int(input())
S = input()
G = {}
H = {}
for i in range(N-1):
    a,b = sorted(list(map(int, input().split(' '))))
    if a-1 in G:
        G[a-1].append(b-1)
    elif a-1 not in G:
        G[a-1] = [b-1]
    if b-1 in H:
        H[b-1].append(a-1)
    else:
        H[b-1] = [a-1]
        
for i in G:
    G[i] = sorted(G[i])
for i in H:
    H[i] = sorted(H[i])

def dfs(graph, node, started, ws, wss):
    if node not in graph:
        if S[node] == 'w':
            ws += 1
        wss.append(ws)
        return
    if S[node] == 'w':
        ws += 1
    else:
        if started:
            started = False
        else:
            wss.append(ws)
            return
    for n in graph[node]:
        dfs(graph, n, started, ws, wss)

def nc2(n):
    if n < 2:
        return 0
    f = factorial
    return f(n) // f(2) // f(n-2)

forward = []
for i in range(len(S)):
    if S[i] == 'c':
        dfs(G, i, True, 0, forward)

backward = []
for i in reversed(range(len(S))):
    if S[i] == 'c':
        dfs(H, i, True, 0, backward)

print(reduce(lambda x,y: x+nc2(y), forward + backward, 0))