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(start, root):
    cw = [0, 0]
    #stack = [(r, start) for r in rinsetsu[start]]
    stack = [(start, root)]
    while len(stack) > 0:
        n, prev = stack.pop()
        cw[0] += S[n] == 'c'
        cw[1] += S[n] == 'w'
        for nn in rinsetsu[n]:
            if nn != prev:
                stack.append((nn, n))
    return cw

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