N = input()
S = raw_input()
rinsetsu = [[] for i in xrange(N)]
for _ in xrange(N-1):
    a, b = map(int, raw_input().split())
    rinsetsu[a-1].append(b-1)
    rinsetsu[b-1].append(a-1)

def dfs(char, n, parent):
    return sum(dfs(char, c, n) for c in rinsetsu[n] if c != parent) + (S[n]==char)

ans = 0
for root in xrange(N):
    if S[root] != 'w':
        continue
    c = [dfs('c', child, root) for child in rinsetsu[root]]
    w = [dfs('w', child, root) for child in rinsetsu[root]]
    for i in xrange(len(rinsetsu[root])):
        for j in xrange(i+1, len(rinsetsu[root])):
            ans += c[i]*w[j] + c[j]*w[i]
print ans